Revert "Re-land removing RECOVERY_AS_BOOT check for init_first_stage"

This reverts commit 4818f748.

The previous commit might introduce a build break on legacy
devices that with a smaller /boot partition size, because it
outputs the first-stage init and the second-stage init binaries
in the following locations, respectively (when
BOARD_USES_RECOVERY_AS_BOOT is true).

    * $OUT/recovery/root/first_stage_ramdisk/init
    * $OUT/recovery/root/system/bin/init

The first one is not needed because there is already a symlink
under $OUT/recovery/root:  init -> /system/bin/init.

Bug: 193565271
Test: `lunch aosp_flame-userdebug`, `make bootimage_debug` then
      checks the output under $OUT/debug_ramdisk/.
Test: Performs the same check for `lunch aosp_bramble-userdebug`.
Change-Id: I6277a6c4c0980bd976cc35db6ca99eafc6bf3f5a