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>

SVN-Revision: 43869