split the env variables correctly
update the set and unset env scripts on each build
Restructure env_script_builder a bit

So that it doesn't treat the directories added for bin lib and
pkg-config differently.

Added directory member to EnvVariable but its not accessible from json
since its semantics is a bit odd. ie. It will only work for the first value.
Add a none target to bss so that its easy to deslect

the current buildshell