Allow custom USB enumeration.

This patch simply checks if NUM_ENDPOINTS is defined, and will bypass the
built-in USB descriptor, device and memory code, and allows a custom setting.
This bug does not affect the IDE unless someone adds the custom option to
the boards.txt file, and provides replacement code in a library or sketch.
It can also be used to turn off USB by providing no replacements, while
allowing speeds >= 20MHz.

Add usb_midi_class::setHandleSysEx()

Switched order of parameters as suggested by PaulStoffregen.

more 2 MHz (VLPR) fixes

resync

fix for 2 and 4MHz

resync

resync

resync

configurable path to toolchain

Use the proper way to refer to gcc's asm extension so we can compile this in C standard mode.

added gitignore file to ignore compile results

+Overclock 144 MHz only

Attention, slower SPI

thank you, duff !