implement exportvariable filter

[smoors]

fixes #281

this implements the approach suggested by @boegel in the issue: if the filter is specified in a comment, file bot/export_vars.sh is created and sourced before running the bot/build.sh script

notes:

• as is the case with other filters, spaces or colons cannot be used
• variables have to be specified one by one, which was easier to implement and avoids having to specify another delimiter, which then also cannot be used
• i used filter name exportvariable instead of the suggested build_env to avoid confusion with other uses of build_env in the code.

example usage:

bot: build arch:zen4 export:MYVAR1=myval1 export:MYVAR2=myval2