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/rbc823/rbc823.c b/board/rbc823/rbc823.c
index e10d9f9..ddfba14 100644
--- a/board/rbc823/rbc823.c
+++ b/board/rbc823/rbc823.c
@@ -127,15 +127,16 @@
 
 int checkboard (void)
 {
-	char *s = getenv ("serial#");
+	char buf[64];
+	int i = getenv_f("serial#", buf, sizeof(buf));
 
-	if (!s || strncmp (s, "TQM8", 4)) {
+	if (i < 0 || strncmp(buf, "TQM8", 4)) {
 		printf ("### No HW ID - assuming RBC823\n");
 		return (0);
 	}
 
-	puts (s);
-	putc ('\n');
+	puts(buf);
+	putc('\n');
 
 	return (0);
 }