Mfleming/more bootloaders

Incorporate two more bootloaders into the project, gummiboot and efilinux. The gummiboot and gnu-efi patches are Stefan's patches from upstream oe-core.

This is ground work for expanding the boottime testing coverage. Note however that these changes don't add the bootloaders into the image recipes just yet, so more work is required to improve the coverage - this is only the first step.