Changes

test: move S1AP PDU samples (blobs) into their own file

This allows using these samples from other (upcoming) tests.

Change-Id: I4b1d3f3a009009ab7620656a3b906e60481e47a1

osmo_s1gw: add set_log_prefix/1

Let's have a common API that can be used by all other modules.

Change-Id: I77ee96f195067ef0137f2cb8af5d455b24ccdd54

sctp_proxy: add _from_{enb,mme} variants of sctp_send/2

The existing sctp_send/2 is sending data from an eNB towards
the MME.  Sending data in the opposite direction is done in-place.

For the sake of readability, let's add the counterpart of sctp_send/2
and clarify the direction by adding _from_{enb,mme} to function names.

Change-Id: I388137e27ea5000f22e7a5b58d1ffcfd9caa1475

sctp_proxy: fix wrong operator in connecting/3

We need to update the map, thus the update operation (`=>`) needs
to be used, not the matching (`:=`).  We're lucky that eNBs usually
do not send anything before the MME responds to the S1Setup request;
otherwise the S1GW would crash here due to a mismatch.

Change-Id: I26d8a8da5cce89324a9e1150e4ecdf2084d097c8