vty: add cmd "reset sgsn state"

Reset the SGSN internal state. Useful when testing the SGSN via TTCN3.
Depends on the libosmocore commit:
I29b6ad6742ddf9b0b58b4af37d9a1cf18e019325

Change-Id: I92096f3f6ea49e75676e30e9921d00210bac5382
1 file changed