Wic gets bitbake variables by parsing output of 'bitbake -e' command.
This implementation improves this procedure as it runs 'bitbake -e' only
when API is called and does it only once, i.e. in a "lazy" way. As parsing
results are cached 'bitbake -e' is run only once and results are parsed
only once per requested set of variables.
get_bitbake_var became the only API call. It replaces find_artifacts,
find_artifact, find_bitbake_env_lines, get_bitbake_env_lines,
set_bitbake_env_lines and get_line_val calls making API much more clear.
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
