efi: runtime-maps: Clarify purpose and enable by default for kexec

The current Kconfig logic for CONFIG_EFI_RUNTIME_MAPS does not convey
that without it, a kexec kernel is not able to boot in EFI mode at all.
So clarify this, and make the option only configurable via the menu
system if CONFIG_EXPERT is set.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Dave Young <dyoung@redhat.com>