A lot fixes for NRF51822. Make pyOCD usable with nRF51 CPU.

- add options for gdb_server.py: SoftDevice flash protection, don't break at hard fault handler, correct resume procedure, add extra monitor command.
- mask interrupt when stepping. Required for NRF51. Otherwise GDB server can't step if any interrupt occured during halted state. In case of nrf51 with enabled BLE it happens all the time.
- fix nrf51 reset(), resetStopResume() functions.
- add nrf51 specific gdb_server server.
How does gdb_nrf differs from the generic gdb? All those new options can be added to the generic demo. I don't see any specific code here.

The conversion to hex integer for flash protect, should happen inside the gdb, not in the application layer. This will enforce all people to do conversion in the application layer?

Shall we name it just flash_offset? It does not protect it anyhow per say, just does not flash first N bytes. Correct?