Assert BOARD_BUILD_SYSTEM_ROOT_IMAGE is not true

Right now BOARD_BUILD_SYSTEM_ROOT_IMAGE is either empty or false.
This change stops new users from setting BOARD_BUILD_SYSTEM_ROOT_IMAGE
to true. After all remaining references (read or set to empty/false) to
BOARD_BUILD_SYSTEM_ROOT_IMAGE are removed, we can mark it as obsolete.

Bug: 241346584
Test: Presubmit build_test
Change-Id: Ie23504525ffc0f0dcad6dff81e7eddb22edffd0e