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).

The HID device must appear before the CDC in order for both to work at
the same time.

Whilst the code is working, it's not currently used.

Adds readline_init() to clear readline history on soft reset.  Addresses
issue #387.

Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].

Added support for the ADC channels and mappings to make_pins.py

I'm not sure if the hal properly deals with the channel 16/18 differences
between the 40x and 42x. It seems to deal with it partially. This particular
aspect will need testing on a 42x or 43x.

Just low-level read/write support.  No filesystem yet.

Tweaked a couple of the USB files to compile if neither dev nor host was defined.
Tested on netduiono plus 2 and stm32f4discovery boards