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.
I assumed that flash offset just moves the flash data, not that it ignores those data. Why did you add this? Why do you need to ignore beginning part of the flashing data?
I understood it previously (without seeing this few lines of code above) that it just offsets them. For a chip with bootloader or just place a binary at exact address (offset).