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/boot/grub2/ostree-grub-generator

@@ -33,6 +33,7 @@ read_config()
     initrd=""
     options=""
     linux=""
+    devicetree=""
 
     while read -r line
     do
@@ -48,6 +49,9 @@ read_config()
             "linux")
                 linux=${value}
                 ;;
+            "devicetree")
+                devicetree=${value}
+                ;;
             "options")
                 options=${value}
                 ;;
@@ -74,6 +78,9 @@ populate_menu()
         if [ -n "${initrd}" ] ; then
             menu="${menu}\t initrd ${boot_prefix}${initrd}\n"
         fi
+        if [ -n "${devicetree}" ] ; then
+            menu="${menu}\t devicetree ${boot_prefix}${devicetree}\n"
+        fi
         menu="${menu}}\n\n"
     done
     # The printf command seems to be more reliable across shells for special character (\n, \t) evaluation