emulator: enable vendor boot image

BUG: 149826779
Test: Build x86, x86_64, armv7 and arm64
make sure x86/64 boot to home screen;
armv7/arm64 can pass disk mounting stage

Change-Id: I258b69bd0cdc4fb0a99b1a04567051e8d32c389a
Merged-In: I258b69bd0cdc4fb0a99b1a04567051e8d32c389a