luci.mk: add simple cache buster

Append a ?v=version string to any media or resource URL that has a .js or .css
extension.

Also substitute placeholders in the form <%# xxxPKG_VERSION %> where
xxx is replaced verbatim and PKG_VERSION with the actual version, e.g.
<%# ?x=PKG_VERSION %> will result in "?x=git-16.237.30043-f885853".

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