Draft: Update Configuration for recent Gluon versions

This patch set solves two build problems I encountered when building Gluon v2023.2.3 with the FreifunkSaar configuration:

• GLUON_FEATURES and GLUON_SITE_PACKAGES are deprecated and have been replaced by image-customization.lua. The patch creates this file and uses the same options that were in site.mk before. See https://gluon.readthedocs.io/en/latest/releases/v2023.2.html#image-customization
• Version number is bumped to 1.13.x

I have successfully built and tested the ipq40xx-generic target.