Remove Kconfig entries, boot subdirectory, dependencies from other
boot-* Makefiles, and sections from ld scripts.

Remove stale redboot code that used to pass initrd addresses in a3 and
a4 to _start.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>