Added an option 'lang' in SConstruct to select
between PicoC and Lua.

A few configurable variables for the languages
are different. For example: 'target' and 'boot'.
So add configurable variables according to the
selected language

+ a few other trivial things.
When lang is 'picoc', the Lua language files are not
compiled. This means that the Lua files need not be
individually protected with #ifdefs.