oe/utils: extract method for parallel_make without d context

oe/utils: extract method for parallel_make without d context

oeqa does not have general access to d variable context and needs to
determine parallel make settings.
Extract the code from parallel_make into reusable parallel_make_value.

Also correct function description of return value from None to empty
string.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>