gs101: re-enable prebuilt boot.img/boot-user.img usage

When $(TARGET_KERNEL_DIR) contains:
* boot.img
* boot-user.img

then use those as prebuilt boot images for the target build.

Note that debug kernels (e.g. $(TARGET_KERNEL_DIR)/kasan) only have
Image.lz4, so the debug build ends up building boot.img from source.

Bug: 199997188
Change-Id: I6bb70644d8ab7356986f6ca6a77f3098d1ee0f83
1 file changed