efi: A misc char interface to update EFI firmware

This patch introduces a kernel module to expose a capsule loader
interface (misc char device file note) for users to upload capsule
binaries.

Example:
cat firmware.bin > /dev/efi_capsule_loader

Any upload error will be returned while doing "cat" through file
operation write() function call.

Tested with Intel Quark Galileo GEN1 platform.

Signed-off-by: Kweh, Hock Leong <hock.leong.kweh@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Peter Jones <pjones@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[ Update comments and Kconfig text ]
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>