travis: Replace pre-built ARM/ARM64 GRUB images with the one built from source

As of today travis uses the pre-built GRUB ARM/ARM64 images from
opensuse. But azure/gitlab are using images built from GRUB 2.04
source. This updates travis to build GRUB ARM/ARM64 UEFI targets
from source, to keep in sync with azure/gitlab.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>