kernel-fitimage.bbclass: fix a wrong conditional check

It should check if "${UBOOT_SIGN_ENABLE}" equals to "1" instead of
checking if "${UBOOT_SIGN_ENABLE}" is not empty since it could be "0".

(From OE-Core rev: 900949af7fe357ee66065ba150b0b1914e8ca581)

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel-fitimage.bbclass

@@ -331,7 +331,7 @@ fitimage_emit_section_config() {
 
 	conf_csum="${FIT_HASH_ALG}"
 	conf_sign_algo="${FIT_SIGN_ALG}"
-	if [ -n "${UBOOT_SIGN_ENABLE}" ] ; then
+	if [ "${UBOOT_SIGN_ENABLE}" = "1" ] ; then
 		conf_sign_keyname="${UBOOT_SIGN_KEYNAME}"
 	fi