Arch-level Kconfig menu cleanup

Remove arch-level Kconfig menu option as it shows all available architectures in
make menuconfig. Instead pull the bootblock options for choice and update image
to top-level Kconfig since it is already present for both x86 and arm.

Change-Id: Iab9c4539f05cd54a7f751565fefcaf7b6f0edc86
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: http://review.coreboot.org/5673
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>