boards/x86: scripts: build_grub.sh to use grub-2.04-rc1-17-g8e8723a6b

Newer versions of GCC (e.g. gcc 9.1.1) fail to compile the version of
Grub that is used by the Zephyr build_grub.sh script. This patch updates
the version of Grub to the latest (as of June 4 2019) which includes a
number of fixes that solve the problem.

Fixes: #16624
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>