Gummiboot was dropped from Alpine. Switch to grub, which is what we use
for tablet-x86uefi too.
In contrary to tablet-x86uefi, run the grub-mkimage command directly
inside the device package:
* This avoids creating another confusing grub-x86 like package (looks
like the x86 version of alpine's grub package, as it's the case with
grub-efi-x86, but actually it is just the result of grub-mkimage).
* In contrary to grub-efi-x86, the resulting efi file is not shared
across several devices.
* It's consistent with uboot mkimage commands, which we also run in
device packages.
Related: 398b7aef0b
