jobs/ttcn3-testsuites-kernel-git: add OSMOCOM_VERSION

Let the user choose whether to run against osmo-ggsn latest or master.
I chose the name "OSMOCOM_VERSION" for the variable to make this easier
to use for people not familiar with the Osmocom stack / the test
infrastructure, as suggested in the related issue.

Related: OS#3208
Change-Id: Ifaf8ed6502b469ade670c3f436670480d27becd6

jobs/ttcn3-testsuites-kernel-git.yml

@@ -55,6 +55,16 @@
                 <code>KERNEL_URL</code> and <code>KERNEL_REMOTE_NAME</code>
                 before building the kernel.
           default: 'master'
+      - choice:
+          name: OSMOCOM_VERSION
+          choices:
+            - latest
+            - master
+          description: |
+                Version of the Osmocom stack to test against the kernel. Either
+                <code>latest</code> for the latest release version, or
+                <code>master</code> for HEAD of git.osmocom.org.
+          default: 'latest'
 
     builders:
       - shell: |-
@@ -65,7 +75,7 @@
           case "{job-name}" in
               ttcn3-ggsn-test-kernel-git)
                   DIR="ttcn3-ggsn-test"
-                  export IMAGE_SUFFIX="master"
+                  export IMAGE_SUFFIX="$OSMOCOM_VERSION"
                   export KERNEL_TEST=1
                   export KERNEL_BUILD=1
                   ;;