docs/fuchsia: update README.md

The build process has changed somewhat since switching to host fuzzing
mode. Also, rearrange the fx build steps earlier, since they're a
prerequisite for building syzkaller.
diff --git a/docs/fuchsia/README.md b/docs/fuchsia/README.md
index 472362c..5c65ea4 100644
--- a/docs/fuchsia/README.md
+++ b/docs/fuchsia/README.md
@@ -9,30 +9,25 @@
 To run syzkaller with a fuchsia target, you will need:
 
 * A fuchsia checkout.
-* The [fuchsia
-  sdk](https://chrome-infra-packages.appspot.com/p/fuchsia/sdk/core/linux-amd64/+/).
-* [clang with fuchsia
-  support](https://chrome-infra-packages.appspot.com/p/fuchsia/clang/linux-amd64/+/).
 
 The rest of the document will use the following environment variables:
 
 * `SOURCEDIR` path of your fuchsia checkout.
-* `FX_SDK_PATH` path where you extracted the fuchsia sdk.
-* `CLANG_PATH` path where you extracted clang.
 
-### Build Go toolchain for fuchsia
+## Building Fuchsia
 
-Syzkaller needs to cross-compile it's Go binaries to run in fuchsia, so first
-you need to compile a Go toolchain that can compile binaries for fuchsia.
+To build fuchsia run:
 
-Once you downloaded and extracted the fuchsia sdk and clang, run:
-
+```shell
+$ fx --dir "out/arm64" set core.arm64 --with-base "//bundles:tools"
+$ fx clean-build
 ```
-$ make TARGETOS=fuchsia TARGETARCH=amd64 \
-    SOURCEDIR=path/to/fuchsia/checkout \
-    FX_SDK_PATH=path/to/fuchsia/sdk \
-    CLANG_PATH=path/to/clang \
-    fuchsia_go
+
+And
+
+```shell
+$ fx --dir "out/x64" set core.x64 --with-base "//bundles:tools"
+$ fx clean-build
 ```
 
 ## Building binaries for fuchsia
@@ -41,9 +36,7 @@
 
 ```
 $ make TARGETOS=fuchsia TARGETARCH=amd64 \
-    SOURCEDIR=path/to/fuchsia/checkout \
-    FX_SDK_PATH=path/to/fuchsia/sdk \
-    CLANG_PATH=path/to/clang
+    SOURCEDIR=path/to/fuchsia/checkout
 ```
 
 ## Running syz-manager
@@ -54,6 +47,8 @@
 $ ${SOURCEDIR}/out/x64.zircon/tools/zbi -o ${SOURCEDIR}/out/x64/fuchsia-ssh.zbi ${SOURCEDIR}/out/x64/fuchsia.zbi --entry "data/ssh/authorized_keys=${SOURCEDIR}/.ssh/authorized_keys"
 ```
 
+Note: This needs to be repeated after each `fx build`.
+
 Run `syz-manager` with a config along the lines of:
 ```
 {
@@ -81,20 +76,7 @@
 
 ## Update syscall and fidl definitions
 
-Syscall descriptions live in the `sys/fuchsia` folder. To update a syscall, you need to modify the `.txt` file that contains it, make sure your new definition matches the one in zircon's [syscalls.abigen](https://fuchsia.googlesource.com/fuchsia/+/master/zircon/system/public/zircon/syscalls.abigen) file. **If the syscall was used in `executor/common_fuchsia.h`, you need to update the usages there as well**. FIDL definitions do not need manual updating because they are extracted automatically when you run make extract, but they require a fuchsia built for each architecture.
-
-To build fuchsia run:
-```shell
-$ fx --dir "out/x64" set core.x64 --with-base "//bundles:tools"
-$ fx clean-build
-```
-
-And
-
-```shell
-$ fx --dir "out/arm64" set core.arm64 --with-base "//bundles:tools"
-$ fx clean-build
-```
+Syscall descriptions live in the `sys/fuchsia` folder. To update a syscall, you need to modify the `.txt` file that contains it, make sure your new definition matches the one in zircon's [syscalls.abigen](https://fuchsia.googlesource.com/fuchsia/+/master/zircon/system/public/zircon/syscalls.abigen) file. **If the syscall was used in `executor/common_fuchsia.h`, you need to update the usages there as well**. FIDL definitions do not need manual updating because they are extracted automatically when you run `make extract`, but they require Fuchsia to be rebuilt for each architecture (see "Building Fuchsia" above).
 
 Once you updated the syscalls definitions, everything can be regenerated by running:
 
@@ -105,6 +87,8 @@
 
 ## How to generate syscall description for FIDL
 
+TODO: This section is out of date.
+
 Syscall descriptions for FIDL are automatically generated as part of `make extract` as described above.
 
 However, if you wish to manually generate syscall descriptions for a given `.fidl` file, do the following.
@@ -112,14 +96,15 @@
 FIDL files should first be compiled into FIDL intermediate representation (JSON) files using `fidlc`:
 
 ```bash
-/fuchsia/out/x64/host_x64/fidlc --json /tmp/io.json --files /fuchsia/zircon/system/fidl/fuchsia-io/io.fidl
+$SOURCEDIR/out/x64/host_x64/fidlc --json /tmp/io.json --files $SOURCEDIR/zircon/system/fidl/fuchsia-io/io.fidl
 ```
 
 Then run FIDL compiler backend `fidlgen` with syzkaller generator, which compiles a FIDL IR file into a syscall description file:
 
 ```bash
-/fuchsia/out/x64/host_x64/fidlgen -generators syzkaller -json /tmp/io.json -output-base fidl_io -include-base fidl_io
+$SOURCEDIR/out/x64/host_x64/fidlgen -generators syzkaller -json /tmp/io.json -output-base fidl_io -include-base fidl_io
 ```
+
 ## Running syz-ci locally
 
 To run `syz-ci` locally for Fuchsia, you need: