simplesoc: free LED
make: add clean action
programmer: add USBBlaster and use platform.bitstream_ext in make
sdramphy: move and clean up s6ddrphy, add generic SDRAM PHY
simple: create PowerOnRst and use it (remove vendor-dependent code)
define platform.soc_ext_path when --external argument is used (to use it to include verilog files in the external target)
tools: replace mkmscimg with mkmscimg.py (mkmscimg.c was platform dependent)
Refactor CRC tools