wic: try to find bitbake using find_executable API
Current wic code was assuming that bitbake directory is on the
same level as scripts, which is not the case for oe classic.
Using find_executable bitbake location should be determined better
as this API uss $PATH to search for executables.
Fixes [YOCTO #7621]
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com> Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
