Changes

Add templates for S1AP Handover procedure

These templates include handover preperation and resource allocation for
S1 handover procedure. It supports mandatory information elements only
at this time.

Related: SYS#7309
Change-Id: Ib8b8aee0665a4f6644287b802f5e941bd82128f6

S1GW tests: Add test cases for S1 handover procedure

The test cases expect the handover preparation and resource allocation
to be passed on correctly with expected modifcation of E-RABs.

Related: SYS#7309
Change-Id: Id349c84749f7e897defa8cfd2ecd2c1f2fb52fc7

mme: Set EPS Attach Type to valid value 'EPS Attach'

Previous value 0 is unused, see TS 3GPP TS 24.301 9.9.3.11.

Change-Id: I7b0c028620ee1ff6b819cce95b39fb93a70d4106

asterisk/gen_links.sh: fix broken symlink

There exist two files named `PIPEasp_Templates.ttcn`:

* library/PIPEasp_Templates.ttcn
* deps/titan.TestPorts.PIPEasp/demo/PIPEasp_Templates.ttcn

We use our own one from the library, not the demo one.

In `asterisk/gen_links.sh` we first create a broken symlink, and then
overwrite it with a valid symlink to `library/PIPEasp_Templates.ttcn`.
This should not be a problem, since `gen_links.inc.sh` is calling ln
with flags `-sf`, yet we saw a build failure in Jenkins:

https://jenkins.osmocom.org/jenkins/view/TTCN3/job/ttcn3-asterisk-ims-ue-test/559/consoleText

------
 > [3/5] RUN	TTCN3-DOCKER-PREPARE "master" asterisk:
0.463 '/ttcn3-dumpcap-stop.sh' -> '/osmo-ttcn3-hacks/ttcn3-dumpcap-stop.sh'
0.463 + ln -sv /osmo-ttcn3-hacks/ttcn3-tcpdump-stop.sh /ttcn3-tcpdump-stop.sh
0.464 '/ttcn3-tcpdump-stop.sh' -> '/osmo-ttcn3-hacks/ttcn3-tcpdump-stop.sh'
0.465 + ln -sv /osmo-ttcn3-hacks/_scripts /_scripts
0.466 '/_scripts' -> '/osmo-ttcn3-hacks/_scripts'
0.466 + make asterisk
0.581 (cd asterisk && ./gen_links.sh && ./regen_makefile.sh)
0.627 ttcn3_makefilegen: error: Cannot find any source file for argument `PIPEasp_Templates.ttcn'.
0.631 ttcn3_makefilegen: warning: TTCN-3 preprocessing (option `-p') is enabled,
                                  but no TTCN-3 files to be preprocessed were given for the Makefile.
0.632 make: *** [Makefile:131: _build/asterisk/Makefile] Error 1
------

Change-Id: Id79bd056716c34c4de02aba68caab95d185e6e48