Build EC as a submodule, specifying version

For models that use System76 EC, we want to have the board version and
the EC version matich.

Ref: https://github.com/system76/firmware-open/issues/75

scripts/build.sh

@@ -4,7 +4,7 @@ set -e
 
 if [ -z "$1" ]
 then
-  echo "$0 [model]" >&2
+  echo "$0 <model>" >&2
   exit 1
 fi
 MODEL="$1"
@@ -79,6 +79,15 @@ KERNELVERSION="${VERSION}" \
         "${MODEL_DIR}/coreboot.config" \
         "${COREBOOT}"
 
+# Rebuild EC firmware for System76 EC models
+if [ ! -e  "${MODEL_DIR}/ec.rom" -a -e "${MODEL_DIR}/ec.config" ]
+then
+    env VERSION="${VERSION}" \
+        ./scripts/_build/ec.sh \
+        "${MODEL_DIR}/ec.config" \
+        "${BUILD}/ec.rom"
+fi
+
 if [ "${MODEL}" != "qemu" ]
 then
     # Rebuild firmware-update