Continuing work on the build system, try 'lua build_elua.lua board=et…

…-stm32 prog' (it only works for stm32 for now). Still a lot of WIP and not throughly tested (for example the new mkfs.lua script was not tested at all with a non-empty FS) but it does seem to work nicely. NOTE: you need to install 'lpack' for this to work (luarocks install lpack). Anybody knows of a good way

to implement make's (or scon's ) '-jn' option (for parallel builds) in a cross-platform manner?