simplesoc: free LED
make: add clean action
programmer: add USBBlaster and use platform.bitstream_ext in make
tools: remove mkmscimg, create crc32.py to replace it and use it in make.py
sdramphy: move and clean up s6ddrphy, add gensdrphy (Generic SDRAM PHY)
simple: create PowerOnRst and use it (remove vendor-dependent code)