The BITBAKEDIR change does not work well when the script is sourced from another script
since $2 may be unrelated. This change adds the logic onto the BDIR conditional and
which more external scripts would set, hence avoiding the problem.
(From OE-Core rev: ec8fbe0d1870285a4a972ddcfe83aa63d720cb80)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>