[packman] add source repository to obs environment

[Diego Ercolani]

Hello, can I ask something about obs environment?
I didn't find the information I need in the online documentation: I would like 
to build linphone against belle-sip 1.4.2 as the actual belle-sip 1.5.0 breaks 
linphone
I built the belle-sip 1.4.2: home:dercol:branches:Multimedia/belle-sip
But, to test linphone I need to build my branch of linphone 
(home:dercol:branches:Multimedia/linphone) against the 
home:dercol:branches:Multimedia/belle-sip and I don't know how to tell obs to 
use home:dercol:branches:Multimedia/belle-sip as source for belle-sip