xtensa: drop CONFIG_EMBEDDED_RAMDISK

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: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>