examples/tests: shell fixes for boards w/o uart0

so far example/test applications using the shell did not work with the 'new' board implementations that do not implement the uart0 board-specific driver. This PR fixes this...

Tested with
- stm32f4discovery
- arduino-due
- avsextrem

This PR also obsoletes #1106.
we should enable getchar to work with uart0:
```c
    char *res = (char*)buffer;
    res[0] = (char)uart0_readc();
    return 1;
```