tablet-x64uefi: use grub instead of gummiboot

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 only not shared
  across several devices.
* It's consistent with uboot mkimage commands, which we also run in
  device packages.

• Related: https://gitlab.alpinelinux.org/alpine/aports/-/commit/398b7aef0b86c477103ba592526ebb9940a44509
• Fix #1095 (closed)

I've verified that this works by flashing an image to a USB stick and booting it on a laptop.