buildtest: dont rely on boards/Makefile.base

before:
```
Building for msba2 .. success
Building for msb-430 .. success
Building for msb-430h .. success
Building for wsn430-v1_3b .. success
Building for wsn430-v1_4 .. success
Building for chronos .. success
Building for native .. success
```

after:
```
$ make buildtest
Building for msba2 .. success
Building for wsn430-v1_3b .. success
Building for olimex_lpc2148 .. failed
Building for msb-430h .. success
Building for chronos .. success
Building for msb-430 .. success
Building for pttu .. failed
Building for native .. success
Building for telosb .. success
Building for redbee-econotag .. failed
Building for wsn430-v1_4 .. success
Building for avsextrem .. success
```