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-uboot.c

@@ -46,11 +46,15 @@ G_DEFINE_TYPE_WITH_CODE (OstreeBootloaderUboot, _ostree_bootloader_uboot, G_TYPE
                          G_IMPLEMENT_INTERFACE (OSTREE_TYPE_BOOTLOADER, _ostree_bootloader_uboot_bootloader_iface_init));
 
 static gboolean
-_ostree_bootloader_uboot_query (OstreeBootloader *bootloader)
+_ostree_bootloader_uboot_query (OstreeBootloader *bootloader,
+                                gboolean         *out_is_active,
+                                GCancellable     *cancellable,
+                                GError          **error) 
 {
   OstreeBootloaderUboot *self = OSTREE_BOOTLOADER_UBOOT (bootloader);
 
-  return g_file_query_file_type (self->config_path, G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS, NULL) == G_FILE_TYPE_SYMBOLIC_LINK;
+  *out_is_active = g_file_query_file_type (self->config_path, G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS, NULL) == G_FILE_TYPE_SYMBOLIC_LINK;
+  return TRUE;
 }
 
 static const char *