Bug 40414: Add common config and functions files

Add common config file used to set Tor Browser version (and later other
things). We also add a `functions` file where we can put functions used
in multiple scripts.

The following lines can be used at the top of a script to use the config
and functions files:

  script_dir=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
  source "$script_dir/functions"

tools/signing/functions

+function exit_error {
+  for msg in "$@"
+  do
+    echo "$msg" > /dev/stderr
+  done
+  exit 1
+}
+
+function var_is_defined {
+  for var in "$@"
+  do
+    test -n "${!var}" || exit_error "$var is not defined (see set-config* files)"
+  done
+}
+
+. "$script_dir/set-config"

tools/signing/set-config

+. "$script_dir/set-config.tbb-version"

tools/signing/set-config.tbb-version

+# The following 3 lines should be uncommented and updated:
+
+#tbb_version=11.5a4
+#tbb_version_build=1
+#tbb_version_type=alpha
+
+var_is_defined tbb_version tbb_version_build tbb_version_type