Add boot.scr snippet for handling the M4

- Check if m4enabled is set. If it's set to 1, attempt to load the
overlay. If that's successful, call the m4boot command to stuff the
binary into memory and kick the core.

Change-Id: Iad8d1ca8f1e04f08b5e747f0dc799f55466eb854
diff --git a/debian/boot.txt b/debian/boot.txt
index 2a2967d..8d6e56d 100644
--- a/debian/boot.txt
+++ b/debian/boot.txt
@@ -49,4 +49,15 @@
 	done
 fi
 
+if printenv m4enabled; then
+	if test ${m4enabled} = 1; then
+		if ext2load mmc ${bootdev}:1 ${fdtovaddr} fsl-imx8mq-phanbell-m4.dtbo && fdt apply ${fdtovaddr}; then
+			echo M4 overlay loaded, starting core
+			run m4boot
+		else
+			echo M4 overlay failed
+		fi
+	fi
+fi
+
 booti ${loadaddr} - ${fdt_addr}