add fast L2 preload, FPGA flashing targets and FPGA Readme info

[da-gazzi]

As the name says:

• I copied Manuel's "fast L2 preloading" from Pulpissimo, which can be activated by passing the LOAD_L2=FAST_DEBUG_PRELOAD parameter and drastically reduces the simulation wait time for loading code into L2
• I added targets for flashing FPGA boards (flash_zcu102/flash_vcu118) and added some more information to the FPGA README.
  FPGA stuff has been tested on ZCU102.