Add apple_set_os command

Hello!
I wanted to bring some attention to this patch:
https://lists.gnu.org/archive/html/grub-devel/2013-12/msg00442.html
I haven been using this patch for months and it works well.

The current generation of dual GPU Macbook Pros turn off the integrated Intel GPU when any other OS than Mac OS X is booted.
This works by MAC OS X identifying itself as a custom EFI protocol. The mentioned patch adds a command (apple_set_os) that fakes this OS identification to make all the hardware accessible.

It would be great if that patch could be merged so that distributions will contain the command without manually patching grub.
I think the command is side-effect free as it just gets registered and then can be run by users who need this quirk for their hardware.

References:
https://github.com/0xbb/gpu-switch#requirements
https://wiki.archlinux.org/index.php/MacBookPro11,x#Getting_the_integrated_intel_card_to_work_on_11.2C3
https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1367109
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=781843

Greetings,
Bruno