Coverity: shadow clone mbed-tls

Avoids some (unnecessary) git content download. It also removes some
code that does the tag checkout and instead it is done at the same
git-clone command.

Change-Id: If690a871578325faa8d6b65d8d80261e771a5077
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/script/tf-coverity/common-def.sh b/script/tf-coverity/common-def.sh
index ea54d18..bcf7547 100644
--- a/script/tf-coverity/common-def.sh
+++ b/script/tf-coverity/common-def.sh
@@ -7,6 +7,10 @@
 
 JENKINS_URL=https://jenkins.oss.arm.com/
 
+# mbed TLS variables
+MBED_TLS_DIR=mbedtls
+MBED_TLS_URL_REPO=https://github.com/ARMmbed/mbedtls.git
+
 # mbed TLS source tag to checkout when building Trusted Firmware with Trusted
 # Board Boot support.
 MBED_TLS_SOURCES_TAG="mbedtls-2.18.0"