Allow ROOTFS to use environment variables

This patch allows ROOTFS to use environment variables, preventing the
need for the full script to be changed in order to use qemu as the
testing platform. This treats ROOTFS the same as ARCH and DEFCONFIG,
both of which are used to select qemu vs uml.

Bug: None
Test: Runs
Change-Id: I30bc244eb011bcac3c18324d8bbd0d7207123258
diff --git a/net/test/run_net_test.sh b/net/test/run_net_test.sh
index 4ae7705..8249767 100755
--- a/net/test/run_net_test.sh
+++ b/net/test/run_net_test.sh
@@ -69,7 +69,7 @@
 NUMTAPINTERFACES=0
 
 # The root filesystem disk image we'll use.
-ROOTFS=net_test.rootfs.20150203
+ROOTFS=${ROOTFS:-net_test.rootfs.20150203}
 COMPRESSED_ROOTFS=$ROOTFS.xz
 URL=https://dl.google.com/dl/android/$COMPRESSED_ROOTFS