Add phony target to run pre-build hook script

Hi,

I was looking for a simple way to version some Arduino code projects automatically. I use 'git describe' and a simple custom script for that purpose.

The one bit missing from Arduino-Makefile is a a pre-build hook to invoke a script.

Here is a quick attempt at a pre-build hook. I am not a Makefile wizard, and I am not sure the pre-build dependency on the $(OBJDIR): target is the best way to handle this. Seems to work though, even with parallel build.

Is this something of interest? Any feedback is welcome

Cheers,
jrid