binconfig.bbclass: Fix path rewriting for staged packages.

binconfig.bbclass: Fix path rewriting for staged packages.
* This bbclass does path rewriting within *-config scripts for packages
being staged. However, it used to expect variable assignments in form foo=bar,
whereas some packages (e.g. libpng) have foo="bar". Fixed to understand both
formats.
* Note: I consider this fix a workaround, as it uses overquoted regexps passed
via command line. Instead, whole rewriting should be reimplemented in Python.