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 sane 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.

MFC after: 1 week