Generate man pages at build time from adoc files
Once we have documentation being build in each project repository separately (see #3385), we want to generate man pages for each repository.
The idea here is to have an .adoc file with all the required content to be used in the man pag, and which is already included in the user manual. Then we need to scripts/makefile to transform this .adoc file into man source format and make it be build and installed when --enable-man is passed. Then, install it in the debian packages (which means we in turn need to modify dh_configure or similar to pass the --enable-man flag, and somehow include the common osmo-gsm-manuals.git in there when sending to OBS).