udoo_neo: Staticize board_string()

Change board_string() function to static because it's being used locally.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/board/udoo/neo/neo.c b/board/udoo/neo/neo.c
index efe8605..cfeed6f 100644
--- a/board/udoo/neo/neo.c
+++ b/board/udoo/neo/neo.c
@@ -176,7 +176,7 @@
 	return fsl_esdhc_initialize(bis, &usdhc_cfg[0]);
 }
 
-char *board_string(void)
+static char *board_string(void)
 {
 	switch (get_board_value()) {
 	case UDOO_NEO_TYPE_BASIC: