This extends the 'Advanced build system tricks' section with a make
script to select the board debugger and serial based on the USB serial
number as reported by the device. This solution is different from the
udev-rule solution as it doesn't require administrative permissions.
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.
