[U-Boot] [RFC] travis-ci: source for grub*.efi

Hello Tom,

currently we extract grub*.efi either from Ubuntu or from Suse packages and use these for test/py/tests/test_efi_loader.py. This implies that we do not have test files for architectures that are not supported by Ubuntu and Suse, i.e. RISC-V. Furthermore any change in the packages may hit us.

One solution is to host the grub*.efi files on gitlab.denx.de. Due to the GPL license this would require also to keep a copy of the GRUB code and the scripts we use to create the grub*.efi files.

Another possibility is to build the files on the fly in Travis like we do for QEMU. But I think this is a waste of build time.

Best regards

Heinrich