cmd: pxe: use strdup to copy config

Replace malloc and strcpy by strdup in
function parse_label_kernel.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/boot/pxe_utils.c b/boot/pxe_utils.c
index b59e959..c613818 100644
--- a/boot/pxe_utils.c
+++ b/boot/pxe_utils.c
@@ -1179,11 +1179,10 @@
 	if (!s)
 		return 1;
 
-	label->config = malloc(strlen(s) + 1);
+	label->config = strdup(s);
 	if (!label->config)
 		return -ENOMEM;
 
-	strcpy(label->config, s);
 	*s = 0;
 
 	return 1;