common: use common grub config for 32b arm

On 32b arm, the grub config entry starts Linux using a zImage. As the
kernel binary is installed under filename Image even on 32b arm to match
the UEFI Shell startup scripts, the grub entry will fail to boot Linux.

As the kernel image name is the only substantial difference, use the common
grub configuration on 32b arm to fix Linux boot.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>