microudp: fix udp txlen

* the preample length 8 is already included in sizeof(struct
ethernet_header)
* the minimum packet size is 64, exluding fcs and including
preample it is 68 (the arp functions are correct)
make.py: make a bit more generic

if parts of the soc are not available or not enables on a platform, do
not use their verilog cores.
spiflash: add read-only variable data width spi flash
etherbone: add etherbone slave (= wishbone master)
lx9crg: add crg for lx9 microboard
software/hello_world: add skeleton
tdcdtc: TDC+DTC wishbone peripheral

either generic or spartan6 ioserdes2 based
software/tdcdtc: add example/test app
memtest: shrink memtest space from 64M to 32M

for smaller platforms like the lx9 microboard
remaining changes for lx9 microboard
lx9 impact script for its embedded closed source jtag
add tdcdtc to lx9