vars: Add "tfgit" procedural module to abstract Git operations

Immediate usecase: we clone tf-m-ci-scripts repo in many places, with
fairly complex parameters, so there's a lot of duplication, and updating
all occurrences is time consuming and error prone. So, let's factor out
that operation into a single function.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I7efb2e5bc7ec8c690eca5a114d6e48cb68ec3559
diff --git a/vars/README b/vars/README
new file mode 100644
index 0000000..a91f710
--- /dev/null
+++ b/vars/README
@@ -0,0 +1,5 @@
+This directory hosts Groovy "procedural" modules of the "trustedfirmware"
+library (while ../src/ hosts Groovy "class" modules).
+
+More info:
+https://www.jenkins.io/doc/book/pipeline/shared-libraries/