Fix incorrect use of getenv() before relocation

A large number of boards incorrectly used getenv() in their board init
code running before relocation.  In some cases this caused U-Boot to
hang when certain environment variables grew too long.
Fix the code to use getenv_r().

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: The LEOX team <team@leox.org>
Cc: Michael Schwingen <michael@schwingen.org>
Cc: Georg Schardt <schardt@team-ctech.de>
Cc: Werner Pfister <Pfister_Werner@intercontrol.de>
Cc: Dirk Eibach <eibach@gdsys.de>
Cc: Peter De Schrijver <p2@mind.be>
Cc: John Zhan <zhanz@sinovee.com>
Cc: Rishi Bhattacharya <rishi@ti.com>
Cc: Peter Tyser <ptyser@xes-inc.com>
diff --git a/board/gw8260/gw8260.c b/board/gw8260/gw8260.c
index 28f5ca9..77a1e1d 100644
--- a/board/gw8260/gw8260.c
+++ b/board/gw8260/gw8260.c
@@ -214,13 +214,13 @@
 /*********************************************************************/
 int checkboard (void)
 {
-	char *str;
+	char buf[64];
+	int i = getenv_f("serial#", buf, sizeof(buf));
 
 	puts ("Board: Advent Networks gw8260\n");
 
-	str = getenv ("serial#");
-	if (str != NULL) {
-		printf ("SN:    %s\n", str);
+	if (i > 0) {
+		printf("SN:    %s\n", buf);
 	}
 	return 0;
 }