Fix things so you can just type "make" and get a micropython build

[mithro]

You should be able to go into the litex directory and just type "make" and get a working micropython. To do this we will need;

• Add a way to configure the micropython build (IE BOARD / TARGET / etc)
• Probably move the gateware+header download stuff into the Makefile (the get-gateware.sh script)
• Figure out how to handle the toolchain + conda stuff needed -- maybe Joel's docker hack?
• Make mkimage.py and the xx.fbi generation tools available outside the HDMI2USB-litex-firmware repo
• Figure out how to flash a board without the HDMI2USB-litex-firmware repo -- probably by using hdmi2usb-mode-switch?