init: clean up the 1st/2nd stage init split

The first split of 1st/2nd stage init went a bit overboard, since it
split these even in the case of the recovery image and system-as-root,
which don't actually need the split.  This change simplifies this a
bit:

system-as-root and recovery have a single combined /system/bin/init
and a symlink from /init to it.

non-system-as-root has a separate first stage init at /init on the
first stage ramdisk and a combined /system/bin/init on system.img.

Two particular benefits from this:
1) Removal of the rsync of TARGET_RAMDISK_OUT to the recovery image
2) Decrease of overall space on the recovery image since it won't have
   a statically linked first stage init

This also unified the various entry points of init to depend entirely
on the arguments passed to it, instead of the hybrid of arguments and
environment variable used previously.

Bug: 80395578
Test: boot both system-as-root and non-system-as-root
Change-Id: Ic2f29b6f56b7defc80eaa0e7cd0c9107e978816f