Fix env variables discard
Fix env variables discard

There's a bug in the stable/2009 OE branch where all environment variables get
discarded before the build process starts regardless of what the user wants.
According to docs, if there's any env variable you would like to use while building you just
need to declare it in the shell before calling bitbake using the following
model:

$ export BB_ENV_EXTRAWHITE="MYVAR MYOTHERVAR"

Note that we skip the '$' from MYVAR and MYOTHERVAR meaning we're just giving
bitbake our variables' names and not their content.

To disable the environment cleanup use:

$ export BB_PRESERVE_ENV

Note that some env variables such as $HOME are preserved by default

Signed-off-by: Nicolas Lebedenco <f10a254f51ad30597289a6e48c0c7975fae6d393@bol.com.br>

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