get_signing_directives: Invert sense of pem file check

To be more consistent with the other check in the function.

Author: tdewey-rpi

host-support/terminal-functions.sh

@@ -137,11 +137,12 @@ get_signing_directives() {
         echo "${CUSTOMER_KEY_PKCS11_NAME} -engine pkcs11 -keyform engine"
     else
         if [ -n "${CUSTOMER_KEY_FILE_PEM}" ]; then
-            if [ ! -f "${CUSTOMER_KEY_FILE_PEM}" ]; then
+            if [ -f "${CUSTOMER_KEY_FILE_PEM}" ]; then
+                echo "${CUSTOMER_KEY_FILE_PEM} -keyform PEM"
+            else
                 echo "RSA private key \"${CUSTOMER_KEY_FILE_PEM}\" not a file. Aborting." >&2
                 exit 1
             fi
-            echo "${CUSTOMER_KEY_FILE_PEM} -keyform PEM"
         else
             echo "Neither PKCS11 key name, or PEM key file specified. Aborting." >&2
             exit 1