wic: fix parsing of 'bitbake -e' output

wic: fix parsing of 'bitbake -e' output

Current parsing code can wrongly interpret arbitrary lines
that are of 'key=value' format as legitimate bitbake variables.

Implemented more strict parsing of key=value pairs using
regular expressions.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>