grub: Backport const qualifier fix for gcc-5

gcc-5 is stricter and complains about const to non-const
conversions, we backport the patch from upstream into 2.00

Change-Id: I17db365fdd253daaa1ab726e2a70ecad0ac7b2ae
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>