Compile Lua scripts with the Lua compiler on the target.

The Lua compiler can now be used from the eLua shell. Its
default output is '/mmc/luac.out'.

To enable the Lua compiler, you must provide an additional
parameter during compilation - 'luac=1' (By default,luac=0)

Eg: scons cpu=at32uc3a0512 toolchain=avr32-gcc luac=1 prog

Please note: One can also use the cross compiler on the PC.