oe-buildenv-internal: hint at specifying bitbake path in error message

oe-buildenv-internal: hint at specifying bitbake path in error message

If you check out OE-Core and then run oe-init-build-env you get an error
about not having bitbake checked out in a "bitbake" subdirectory,
however it's possible to specify the bitbake path on the
oe-init-build-env command line, so hint at that in the error message
rather than implying it has to be in the default location.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>