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 crc_inserter.py (mkmscimg was platform dependent)