Gen7-ARM: make this primitive sample code work.

This works now. You can add printf() and even scanf() and it works
over the serial line.

Drawback is, it's no longer self-sufficient, but requires the mbed
library from https://github.com/mbedmicro/mbed . That's a library
officially supported by ARM and tries, mostly successful, to
support all Cortex-M MCUs from all vendors.

Note: forgot to commit the Makefile last time and have no backup
      either, so that's lost.