env: Move env_get() to env.h

Move env_get() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

arch/arm/lib/bootm.c

@@ -15,6 +15,7 @@
 #include <command.h>
 #include <dm.h>
 #include <dm/root.h>
+#include <env.h>
 #include <image.h>
 #include <u-boot/zlib.h>
 #include <asm/byteorder.h>