bootconfig: Move BOOTCONFIG_FOOTER_SIZE to include/linux/bootconfig.h

BOOTCONFIG_FOOTER_SIZE was defined locally in tools/bootconfig/main.c.
Move it to include/linux/bootconfig.h so that it can be shared with
the kernel and user-space tools. Also, use it in init/main.c instead
of the hardcoded (BOOTCONFIG_MAGIC_LEN + 8) expression when checking
the initrd size.

Link: https://lore.kernel.org/all/178913601321.248794.4786502771058559887.stgit@devnote2/

Suggested-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Sang-Heon Jeon <ekffu200098@gmail.com>