boot/grub: fix grub-mkimage with binutils >= 2.31

boot/grub: fix grub-mkimage with binutils >= 2.31

While using pc_x86_64_efi_defconfig with a bleeding-edge x86_64 toolchain
based on binutils 2.31, the grub-mkimage command return this error:

grub-mkimage: error: relocation 0x4 is not implemented yet.

Backport an upstream patch [1] as suggested on openembedded mailing list [2].

[1] http://git.savannah.gnu.org/cgit/grub.git/commit/?id=842c390469e2c2e10b5aa36700324cd3bde25875
[2] http://lists.openembedded.org/pipermail/openembedded-core/2018-July/153054.html

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>