[yocto] Problem with applying a patch using default -pnum

[Hans Beckérus]

Hi. More problems ;)
I have a patch file that needs to be applied to a source tree and the
patch file is copied properly to the ${WORKDIR} directory.
So far so good. But, the problem with this source tree is that it is
not built from the traditional root folder of the repo.
This means I need to change ${S} to point somewhere else. This also
causes the patch system to fail!
I did an override of do_patch() in my .bb and that seems to work, but
I do not like to use overrides unless I really have to.
So basically, is there some way to tell the built-in patch system to
use a different -pnum value?
If there is, I could stick with the do_patch() as provided by default.

Hans