Observed:

1) Using a JeOS rootfs tarball with a TFTP-booted kernel on an AArch64 board
results in a fatal error dialog, complaining about not being able to read
/etc/default/grub. This can be worked around by installing grub2-arm64-efi.

2) After grub2 is installed it will show a fatal error dialog complaining that
/boot/grub2/grub.cfg cannot be read. This can be worked around by
grub2-mkconfig -o /boot/grub2/grub.cfg.

3) With grub2 installed and grub.cfg in place, it will start up normally.

Expected:

a) Absence of grub2 packages should be ignored. When selecting grub2 in the
YaST UI install, the necessary packages should be installed, as done for
others.

b) If grub.cfg doesn't exist yet, it should be generated.