layer.conf: fix sanity error for PATH variable in extensible SDK workflow

Sanity checker reports following error for the PATH variable,
when bitbake -e <recipe> command is run in an extensible SDK workspace.
   PATH contains '.', './' or '' (empty element), which will break the build

In case of extensible SDK, PATH variable is formed with two consecutive ':'
as bb.utils.which(d.getVar('PATH'),'bitbake') call returns an empty string.

This change adds ':' if BITBAKEPATH is a non empty string.

Signed-off-by: Sourabh Banerjee <sbanerje@codeaurora.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>