split first stage init into a separate executable

In the future, systems with dm-linear will require a ramdisk to set up
the mount for system.  In this world, first stage init will be a part
of this ramdisk and handle setting up dm-linear, mounting the
necessary partitions, then pivoting to the system image, which will
become the root partition.

This also enables previous devices without system-as-root, to be
unified with system-as-root devices for all aspects of boot after the
pivot_root.

Bug: 79758715
Test: boot hikey
Test: boot sailfish, boot sailfish into recovery
Change-Id: Iefa88a3ec5994e7989aa9f26f2de0351ffa5468b