cpu/arm/k60: Add a debugging helper hard fault handler.

This is basically a copy and paste of the hardfault handler originally
written by Joseph Yiu and published on various forums around the web.

When a hardfault occurs the new fault handler will try to dump
debugging information to the UART terminal.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>