efi: clean up Kconfig dependencies on CONFIG_EFI

Geert reports that the new option CONFIG_EFI_DISABLE_RUNTIME is user
visible even when EFI support is disabled, which is unnecessary and
clutters the Kconfig interface.

So let's move this option into the existing Kconfig submenu that already
depends on CONFIG_EFI, and while at it, give some other options the same
treatment.

Also clean up a small wart where the efi/ subdirectory is listed twice.
Let's just list it unconditionally so that both EFI and UEFI_CPER based
pieces will be built independently (the latter only depends on the
former on !X86)

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>