Currently, the pre/post process command variables are parsed as shell, even
though they're not shell anymore. As a result, an empty SDK_PACKAGING_FUNC
results in a parsing error. Rather than manually adding their vardeps, only
append its ; when the var is set.
Signed-off-by: Christopher Larson <chris_larson@mentor.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>