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-bootloader.c

@@ -30,11 +30,14 @@ _ostree_bootloader_default_init (OstreeBootloaderInterface *iface)
 }
 
 gboolean
-_ostree_bootloader_query (OstreeBootloader  *self)
+_ostree_bootloader_query (OstreeBootloader *self,
+                          gboolean         *out_is_active,
+                          GCancellable     *cancellable,
+                          GError          **error)
 {
   g_return_val_if_fail (OSTREE_IS_BOOTLOADER (self), FALSE);
 
-  return OSTREE_BOOTLOADER_GET_IFACE (self)->query (self);
+  return OSTREE_BOOTLOADER_GET_IFACE (self)->query (self, out_is_active, cancellable, error);
 }
 
 /**