pyOCD/gdbserver: Fixed XPSR variable name

'IPSR' was used into the code while it was reading 'XPSR'.
pyOCD/gdbserver: Fixed index out of range in lastSignal()

It happens when the debugger had been stopped in a interrupt
handler.
Merge pull request #15 from oliviermartin/gdbserver-ipsr-fixes

Fixes for GDB Server
Merge pull request #16 from xiongyihui/master

add script to flash nrf51822
Corrected so that unsupported mbed devices will be detected, and a message printed to the log. Rather than pyOCD raising an exception and dying.
Merge pull request #17 from DonMorr/master

Fix handling of unsupported mbed devices
flash_nrf51822.py uses intelhex library instead of arm-none-eabi-objcopy to deal with hex file