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• GLUON_FEATURES and GLUON_SITE_PACKAGES are deprecated and have been replaced by image-customization.lua.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
• The key mesh_vpn.tunneldigger.mtu needs to be set explicitly. I followed the recommendation from https://gluon.readthedocs.io/en/latest/user/mtu.html and set it to 1312 for all seven domains.

Note, the version number should probably be bumped as well. I have successfully built and tested the ipq40xx-generic target.