loader: tftp client should use server address from rootip
servip is set from bootp bp_siaddr (if present) and rootip is set immediately from servip in tha same bootp code.
However, the common/dev_net.c does only set rootip (based on url processing etc). Therefore, we should also use rootip in tftp reader.
Fixes hung tftp based boot when bp_siaddr is not provided.
Testing done: tftp boot.