efi: Add misc char driver 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.

Signed-off-by: Kweh, Hock Leong <hock.leong.kweh@intel.com>
[ Update comments and Kconfig text ]
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Reviewed-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Jones <pjones@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: joeyli <jlee@suse.com>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1461614832-17633-30-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/Kconfig

@@ -102,6 +102,16 @@ config EFI_BOOTLOADER_CONTROL
 	  bootloader reads this reboot reason and takes particular
 	  action according to its policy.
 
+config EFI_CAPSULE_LOADER
+	tristate "EFI capsule loader"
+	depends on EFI
+	help
+	  This option exposes a loader interface "/dev/efi_capsule_loader" for
+	  users to load EFI capsules. This driver requires working runtime
+	  capsule support in the firmware, which many OEMs do not provide.
+
+	  Most users should say N.
+
 endmenu
 
 config UEFI_CPER