wic: try to find bitbake using find_executable API

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]

(From OE-Core rev: 9d86eb2e448263f7e395b48d95c380b2dc66ad8e)

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>