Added OS version data and moved varfuncs to base

Two new varfunc categories - os and uecide. OS contains the os name,
version, flavour and arch.  UECIDE contains the UECIDE version and
the product name.

The string parsing routines and the varfuncs have been moved completely to
the Base object so they can be used from other places such as plugins without
any sketch being available.  The plugin manager now uses it for filling
in the arguments to the plugin URL.