Import Netflix's efibootmgr to help manage UEFI boot variables

efibootmgr manages the UEFI BootXXXX variables that implement the UEFI
Boot Manager protocol defined in the UEFI standards. It is modeled
after the Linux program of the same name with a mostly compatible set
of command line options. Since there's a fair amount of OS specifioc
code due to differeing names and methods of doing things, the
compatibility isn't 100%.

Basic functionality is implemented, though the more advanced next boot
functionality that's been defined elsewhere is unimplemented.

Submitted by: Matt Williams (with unix / efi path xlate by me)
Sponsored by: Netflix