efibc: Add EFI Bootloader Control module

This module installs a reboot callback, such that if reboot() is invoked
with a string argument NNN, "NNN" is copied to the "LoaderEntryOneShot"
EFI variable, to be read by the bootloader.

If the string matches one of the boot labels defined in its configuration,
the bootloader will boot once to that label.  The "LoaderEntryRebootReason"
EFI variable is set with the reboot reason: "reboot", "shutdown".

The bootloader reads this reboot reason and takes particular action
according to its policy.

There are reboot implementations that do "reboot <reason>", such as
Android's reboot command and Upstart's reboot replacement, which pass
the reason as an argument to the reboot syscall.  There is no
platform-agnostic way how those could be modified to pass the reason
to the bootloader, regardless of platform or bootloader.

Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stefan Stanacar <stefan.stanacar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1461614832-17633-26-git-send-email-matt@codeblueprint.co.uk


Signed-off-by: Ingo Molnar <mingo@kernel.org>