extensions: grub-with-dtb: add UMS menu entry to GRUB if initramfs ums hook is detected

- this allows user to enter UMS mode from GRUB
- for usage with the `initramfs-usb-gadget-ums` extension
- has no effect unless `initramfs-usb-gadget-ums` extension is enabled together with `grub-with-dtb`

packages/blobs/grub/09_linux_with_dtb.sh

@@ -503,6 +503,10 @@ while [ "x$list" != "x" ]; do
 			"${GRUB_CMDLINE_LINUX_RECOVERY} ${GRUB_CMDLINE_LINUX}"
 	fi
 
+	if [ -f /etc/initramfs-tools/scripts/init-premount/usb-gadget-ums.sh ]; then
+		linux_entry "${OS}" "${version}" init-ums "initrd=ums ums=yes ${GRUB_CMDLINE_LINUX}"
+	fi
+
 	list=$(echo $list | tr ' ' '\n' | fgrep -vx "$linux" | tr '\n' ' ')
 done