buildscript: build dynamic version based on tags

Signed-off-by: Tim Niemeyer <tim.niemeyer@mastersword.de>

buildscript

@@ -34,8 +34,14 @@ prebuild() {
 	#insert actual firware version informations into release file
     dirty=$(test "$(git status  --porcelain -uno)" = "" || echo "-dirty")
     commit=$(git log -n1 | grep commit | awk '{ print $2 }')
-	echo "FIRMWARE_VERSION=\"$commit$dirty\"" > $target/files/etc/firmware_release
-	echo "RELEASE_DATE=\"unreleased\"" >> $target/files/etc/firmware_release
+    version=$(git tag --contains $commit~~ | tail -n1)
+    if [ "$version" = "" ]
+    then
+        version=$commit$dirty
+    fi
+
+	echo "FIRMWARE_VERSION=\"$version\"" > $target/files/etc/firmware_release
+	echo "RELEASE_DATE=\"build date: "`date`"\"" >> $target/files/etc/firmware_release
 	echo "FIRMWARE_REVISION=\"build date: "`date`"\"" >> $target/files/etc/firmware_release
 	echo "OPENWRT_CORE_REVISION=\""`svn info $target |grep Revision: |cut -c11-`"\"" >> $target/files/etc/firmware_release
 	echo "OPENWRT_FEEDS_PACKAGES_REVISION=\""`svn info $target/feeds/packages |grep Revision: |cut -c11-`"\"" >> $target/files/etc/firmware_release