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/arch/arm/mach-meson/board-common.c b/arch/arm/mach-meson/board-common.c
index 18383f7..08a7aae 100644
--- a/arch/arm/mach-meson/board-common.c
+++ b/arch/arm/mach-meson/board-common.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <asm/arch/boot.h>
+#include <env.h>
 #include <linux/libfdt.h>
 #include <linux/err.h>
 #include <environment.h>