GTP_Emulation: make GTPU optional

When using the GTP_Emulation connection handler one has to configure a
GTPC and a GTPU link. However in some situations (e.g. when testing the
Gn interface of an 5g MME) only GTPC may be required. So lets make the
GTPU link optional.

Related: OS#5760
Change-Id: I509a229fcaf02ea5149df42816af27bba46d3bff
1 file changed