configs/pc_x86_64_efi: use genimage GPT partition table support

configs/pc_x86_64_efi: use genimage GPT partition table support

Thanks to the introduction of GPT partition table support in genimage,
this commit improves the pc_x86_64_efi_defconfig to remove the use of
the custom script creating the image.

Tested in QEMU, not on a physical device.

So:
- revert commit fee29b05bb7db25e37c8a5175ce00dc712554edf[1]
- add GPT support
- tweak shell script to add the correct UUID in genimage config.

[1]: https://git.buildroot.net/buildroot/commit/?id=fee29b05bb7db25e37c8a5175ce00dc712554edf
[2]: https://git.buildroot.net/buildroot/commit/?id=79b8540d624ac4846ba341b1b9691eccacf0bc05

Signed-off-by: Alexandre PAYEN <alexandre.payen@smile.fr>
Cc: Carlos Santos <casantos@datacom.com.br>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Thomas:
 - drop commented code in post-build.sh
 - take into account comments made by Carlos Santos in
   http://patchwork.ozlabs.org/patch/1143502/]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>