env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/gumstix/pepper/board.c b/board/gumstix/pepper/board.c
index ea29dcd..7ae4c5c 100644
--- a/board/gumstix/pepper/board.c
+++ b/board/gumstix/pepper/board.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <errno.h>
 #include <spl.h>
 #include <asm/arch/cpu.h>