ptxdist: make icecc handling more robust

Create the reference link last and abort on error.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
net-snmp: add security patch from upstream 5.7.2.1

Upstream 5.7.2.1 was released fixing one security issue (possible denial
of service with ICMP-MIB on linux). However the tarball is broken, so we
just add this one patch on top of the working 5.7.2 tarball.