list-packageconfig-flags.py: fix searching bitbake module failed

list-packageconfig-flags.py: fix searching bitbake module failed

Run list-packageconfig-flags.py on wrlinux's platform in which
the oe-core layer and bitbake layer in different directories:
----
../layers/oe-core/scripts/contrib/list-packageconfig-flags.py
Traceback (most recent call last):
  File "../layers/oe-core/scripts/contrib/list-packageconfig-flags.py", line 28, in <module>
    import bb.cache
ImportError: No module named bb.cache
----

The script import bb module from bitbake lib dir, the previous
lib dir was hardcode and only worked on poky but not for others.

In this situation, look for bitbake/bin dir in PATH could fix this issue.

[YOCTO #5060]

(From OE-Core rev: 9e749c430f97b1a30cdf0c13dacd2a985ef7b433)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>