feat(sys-kernel/x86_64_defconfig) enable EFI in the kernel build

This builds the kernel as an EFI image, allowing it to be booted
directly by UEFI bioses.  It also enables the efivars and efivarfs
modules so that EFI variables can be accessed properly when booted in
EFI mode.
Merge pull request #181 from gregkh/kernel_update

feat(sys-kernel/x86_64_defconfig) enable EFI in the kernel build