BOARD_PREBUILT_BOOTIMAGE: removes the restriction of non system-as-root

Starting from Android 10, the system.img layout consists of
$TARGET_SYSTEM_OUT and $TARGET_ROOT_OUT, and is mounted by the
init as root. That is, system.img is always created as if
BOARD_BUILD_SYSTEM_ROOT_IMAGE was set.

  https://source.android.com/devices/bootloader/partitions/system-as-root

The previous concern is that there might be compatibility issues between
the ramdisk contained in boot.img with a newer system.img. But this is
no longer an issue after we always mount the system.img as root.

Bug: 187157581
Test: Tree Hugger
Change-Id: I4537e6ce6fb39b4b86caac82a13716abf515ffd6