bin/bitbake: Add better environmental variable handling. By default it will now only...

bin/bitbake: Add better environmental variable handling. By default it will now only pass certain whitelisted variables into the data store. If BB_PRESERVE_ENV is set bitbake will use all variable from the environment. If BB_ENV_WHITELIST is set, that whitelist will be used instead of the internal bitbake one. Alternatively, BB_ENV_EXTRAWHITE can be used to extend the internal whitelist.