Beaglebone

The implemented features:

U-Boot setup
UART driver
GPIO driver
Global interrupt enabling/disabling
Preparing a stack and starting a thread
Thread yield
```diff
-export TERMPROG = $(RIOTBASE)/dist/tools/pyterm/pyterm.py
+export TERMPROG = $(RIOTBASE)/dist/tools/pyterm/pyterm
+export TERMFLAGS = -p
```