beagle_x15: Fix boot_fit.img generation

Although bootfitimage module is added in PRODUCT_PACKAGES, it's not
being built on "make" command, and subsequent "make bootfitimage"
invocation is necessary. Let's make bootfitimage a prerequisite of
droidcore rule instead of referencing it in PRODUCT_PACKAGES. This way
boot_fit.img is guaranteed to be built on "make" command.

This is pretty much the same as it's done for HiKey board here:

    device/linaro/hikey/build/tasks/dtimage.mk

but in our case we add phony module name to droidcore prerequisites, not
file path. This way we can still run 'make bootfitimage' separately if
we want to.

Change-Id: I09fa9854437f64d35400311eadb6cdaec46fb144
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/boot_fit/Android.mk b/boot_fit/Android.mk
index fed87fa..225e2d4 100644
--- a/boot_fit/Android.mk
+++ b/boot_fit/Android.mk
@@ -33,5 +33,7 @@
 LOCAL_ADDITIONAL_DEPENDENCIES := $(BOOTIMG_FIT)
 include $(BUILD_PHONY_PACKAGE)
 
+droidcore: bootfitimage
+
 endif
 endif
diff --git a/device.mk b/device.mk
index 527513e..eb47418 100644
--- a/device.mk
+++ b/device.mk
@@ -139,9 +139,6 @@
 	netutils-wrapper-1.0 \
 	messaging \
 
-PRODUCT_PACKAGES += \
-	bootfitimage
-
 # Boot control
 PRODUCT_PACKAGES += \
 	bootctrl.am57x \