dualboot: add missing altboot.txt needed by dualboot

Once you enabled dualboot in the ccmp15 you need this file
for the altbootcmd.

Signed-off-by: Francisco Gil <francisco.gilmartinez@digi.com>

meta-digi-arm/recipes-bsp/u-boot/u-boot-dey/ccmp15-dvk/altboot.txt

@@ -0,0 +1,27 @@
+#
+# U-Boot bootscript for altbootcmd (dual boot fallback after retries)
+#
+
+# After an upgrade, active_system has changed. U-Boot tries to boot this system
+# for a number of tries. If the limit is reached, altbootcmd is run instead.
+# This is the script that it will run. It has to:
+#  * switch back to previous system
+#  * reset the firmware update flag
+#  * run the regular boot command
+
+if test "${dualboot}" = "yes" && test "${upgrade_available}" = "1"; then
+	echo "## Update failed; Rolling back to previous version."
+	if test "${active_system}" = "linux_a"; then
+		setenv active_system linux_b
+		setenv mtdbootpart ${active_system}
+		setenv rootfsvol ${rootfsvol_b}
+	else
+		setenv active_system linux_a
+		setenv mtdbootpart ${active_system}
+		setenv rootfsvol ${rootfsvol_a}
+	fi
+	setenv upgrade_available
+	setenv bootcount 0
+	saveenv
+fi
+run bootcmd