Fix GPT partition creation when the disk is MBR
Uppercase for better compatibility (use UEFI_SYSTEM as label)
fat16 and fat32 should be mounted as vfat in fstab
Add "/boot/efi" mountpoint in the mountpoint combobox when in uefi mode
uefi needs a efi partition mounted at /boot/efi so dont let the advanced
installation continue without it
Put the right boot and boot_legacy flags in uefi mode