build: add version number to filenames

This commit introduces a new option CONFIG_VERSION_FILENAMES which causes
OpenWrt to embed the version number in generated image files, SDK- and
ImageBuilder archives.

The option is enabled by default if CONFIG_VERSIONOPT is set.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43869 3c298f89-4303-0410-b956-a3cf2f4a3e73