usb and sdcard sharing improvements

* Add traceback after watchdog timeout

Add the cpability to perform a traceback following a watchdog timeout.

* Enhanced hardware SPI

Allow use of either SSP0 or SSP1.
Ensure that no data is left in I/O buffers after calls to enable sharing of SSP hardware.

* Make flash emulation of eeprom the default

Make use of flash for eeprom storage the default. This means that usage of eeprom will not cause USB drive mount/unmount operations.

* Allow sharing of SD card

SD card I/O operations from the USB stack take place in idle loop, rather than at interrupt time. Allowing sharing of the SPI bus.

New configuration options to allow usage of the SD card to be specified.

* Fix problem with hardware SPI pins