add usb gamepad to gameboy emulator

[BlitzCityDIY]

add usb gamepad support to the gameboy pico2 emulator.

usb hid report:

default input (no buttons pressed): 00 7F 7F 00 80 80 0F 00 00
a: 00 7F 7F 00 80 80 2F 00 00
b: 00 7F 7F 00 80 80 4F 00 00
x: 00 7F 7F 00 80 80 1F 00 00
y: 00 7F 7F 00 80 80 8F 00 00
up: 00 7F 00 00 80 80 0F 00 00
down: 00 7F FF 00 80 80 0F 00 00
left: 00 00 7F 00 80 80 0F 00 00
right: 00 FF 7F 00 80 80 0F 00 00
start: 00 7F 7F 00 80 80 0F 20 00
select: 00 7F 7F 00 80 80 0F 10 00
r: 00 7F 7F 00 80 80 0F 02 00
l: 00 7F 7F 00 80 80 0F 01 00

first index seems to be ignored, so the 2nd [1] index is actually [0]

[BlitzCityDIY]

@ladyada - added support for the gamepad in the shop to the gameboy emulator