Add definition for helper function "upon"

When running coverity builds locally using the run_coverity_on_tf.py
script, we seeing the following issue:

./tf-a-ci-scripts/script/tf-coverity/common-def.sh: line 63: upon: command not found

The upon function is already being used in common-def.sh file in the
function set_armclang_toolchain. Hence, imported its definition from utils.sh

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I238ca0b60f42c767e3564c55adb5e20f8a4a918b
diff --git a/script/tf-coverity/common-def.sh b/script/tf-coverity/common-def.sh
index f2241ec..eca7080 100644
--- a/script/tf-coverity/common-def.sh
+++ b/script/tf-coverity/common-def.sh
@@ -45,6 +45,14 @@
     return 1
 }
 
+# Use "$1" as a boolean
+upon() {
+	case "$1" in
+		"" | "0" | "false") return 1;;
+		*) return 0;;
+	esac
+}
+
 # Provide correct linaro cross toolchain based on environment
 set_cross_compile_gcc_linaro_toolchain() {
     local cross_compile_path="/home/buildslave/tools"