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-sysroot-private.h

@@ -75,7 +75,10 @@ _ostree_sysroot_get_devino (GFile         *path,
 
 char *_ostree_sysroot_join_lines (GPtrArray  *lines);
 
-OstreeBootloader *_ostree_sysroot_query_bootloader (OstreeSysroot         *sysroot);
+gboolean _ostree_sysroot_query_bootloader (OstreeSysroot     *sysroot,
+                                           OstreeBootloader **out_bootloader,
+                                           GCancellable      *cancellable,
+                                           GError           **error);
 
 G_END_DECLS