Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.

Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected.  Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).

Minor cleanup.

Revamp usrsw to use new exti code.
Add user switch callback function.

This also fixes up the IRQ for the PYBOARD4 USERSW
although I was unable to test that functionality.

* Add some generic feature macros to mpconfigport
* Move pin and port definitions from usrsw.c to mpconfigport

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

* Add a TARGET definition to Makefile (default PYBOARD).
* Add support for discovery LEDs in led module.
* Add support for discovery user switch in usersw
* Add EXTI interrupt handler for discovery user switch on (PA0).
* Parameterize led and usrsw modules pins and port.
* Issue #83

* Move user switch code from main.c into a separate module (usrsw)
* Add usrsw.c to Makefile