build-grub: adapt to changes in build of grub

Commit 35b90906 ("gnulib: Upgrade Gnulib and switch to bootstrap tool")
requires the bootstrap tool to be executed before the configure step.

Change-Id: If73392da4a1605dd61c9b0dce4630829a8e17f7b
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>