Allow BOARD_USES_RECOVERY_AS_BOOT without BOARD_BUILD_SYSTEM_ROOT_IMAGE

The recovery image already contains a copy of first stage init, so we
can boot unconditionally to the recovery image and instruct first
stage init whether or not to boot to Android or to recovery.  In this
case, we need neither the kernel to mount /system as / nor a separate
partition for recovery, so this change modifies the build scripts to
allow this combination.

Bug: 114062208
Test: Boot pixel from recovery to Android with BOARD_USES_RECOVERY_AS_BOOT
      But without BOARD_BUILD_SYSTEM_ROOT_IMAGE
Change-Id: Icd047afb7f22d2724b3bcaca1aa0c837426dcce7