Add (non-atomic) support for GRUB2 + UEFI

We need basic support for UEFI - many newer servers don't support
BIOS compatibility mode anymore.

However, this patch only implements non-atomic because UEFI is FAT, and
we can't do the previous design for OSTree of atomic swap of
/boot/loader.

The Fedora/RHEL UEFI layout has the kernels on a "real" /boot
partition, and /boot/efi/EFI/$vendor just holds the grub2 UEFI binary
and grub.cfg.

Following this, /boot/loader is still on the OS boot partition, and we
still atomically swap it.  This potentially paves the way to atomic
upgrades in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=724246

src/libostree/ostree-cmdprivate.c

@@ -29,9 +29,7 @@
 static gboolean 
 impl_ostree_generate_grub2_config (OstreeSysroot *sysroot, int bootversion, int target_fd, GCancellable *cancellable, GError **error)
 {
-  gs_unref_object OstreeBootloaderGrub2 *grub2 = _ostree_bootloader_grub2_new (sysroot);
-
-  return _ostree_bootloader_grub2_generate_config (grub2, bootversion, target_fd, cancellable, error);
+  return _ostree_bootloader_grub2_generate_config (sysroot, bootversion, target_fd, cancellable, error);
 }
 
 /**