Products launching with API 28 needs system-as-root

With project Treble, it's possible to only update system.img across
Android major releases without changing other partitions. However,
for non-A/B devices, ramdisk cannot be updated via a system-only OTA
because it resides in /boot partition.

This leads to two major incompatibility issues:
  - The older /init in ramdisk might not be able to parse the *.rc files
    on /system.
  - The ramdisk contains /init.rc, which also may be out of date compared to
    what is required for the new /system.

Therefore, for new devices launched in P with full Treble support, ramdisk
should be merged into system.img by setting
BOARD_BUILD_SYSTEM_ROOT_IMAGE := True.

Upgrading devices can keep using non-system-as-root configuration.

Bug: 64194858
Test: manual makefile prodding
Change-Id: Ic04c63bd2aac3afa686a6f5cb124beebbcd6a5ee