grub2: add support for devicetree

Similar as available for u-boot (ce2995e1dc)
and syslinux (c5112c25e4), enable parsing
and writing devicetree filename into grub.cfg.

This is required by arm64-based devices running edk2 instead of u-boot
as the main bootloader (e.g. 96boards HiKey and HiKey960).

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>

Closes: #1790
Approved by: cgwalters

src/libostree/ostree-bootloader-grub2.c

@@ -191,6 +191,7 @@ _ostree_bootloader_grub2_generate_config (OstreeSysroot                 *sysroot
       const char *options;
       const char *kernel;
       const char *initrd;
+      const char *devicetree;
       char *quoted_title = NULL;
       char *uuid = NULL;
       char *quoted_uuid = NULL;
@@ -246,6 +247,15 @@ _ostree_bootloader_grub2_generate_config (OstreeSysroot                 *sysroot
           g_string_append_c (output, '\n');
         }
 
+      devicetree = ostree_bootconfig_parser_get (config, "devicetree");
+      if (devicetree)
+        {
+          g_string_append (output, "devicetree");
+          g_string_append_c (output, ' ');
+          g_string_append (output, devicetree);
+          g_string_append_c (output, '\n');
+        }
+
       g_string_append (output, "}\n");
     }