doc/advanced-build-system-tricks.md: add a doc for new variables

Add a documentation page for advanced low level tricks.
Document the `RIOT_MAKEFILES_GLOBAL_PRE` and
`RIOT_MAKEFILES_GLOBAL_POST`.

I could not work around to have `$(RIOTBASE)/Makefile.include` in the
doc as the `$()` part was removed, so I kept with `$RIOTBASE` for now.