Improving the build system

[jiixyj]

Currently, each time you run 'make' the whole program is recompiled. Is there a particular reason for this behavior?
In my branch I fixed this at least for the Contiki and Shawn make files, see here. Automatic header dependency checking also works, I more or less followed the GNU make manual.

[gitbaum]

Thanks for improvement. No, there is no particular reason. Actually, its only "historical reasons"... Since the re-compiling only affects a couple files (usually not more than 5), I just didn't spend the time for the improvement.

I only see a minor problem in your Makefile (should be easily fixed):

• CXX = avr-g++: for sky (or msb) this must be msp430-g++
• same for avr-objcopy/avr-size -> msp430-*

But then it would be nice if you'd push your Makefile to Wiselib's master (after at least testing it for target contiki_sky, please; if you don't have msp430-g++ installed, you can do it in the VM)

And thanks again for the improvment!