build: support adding version code to file names (FS#323)

Now that the VERSION_NUMBER variable holds the human friendly name and not
the commit ID anymore, we need to support adding the revision ID as well.

Introduce a new config variable CONFIG_VERSION_CODE_FILENAMES which, if set,
causes the resulting file names to contain a commit ID designation as printed
by scripts/getver.sh.

Also sanitize the input variables to ensure that the resulting strings are
lowercased and no not contain spaces.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>