env: Move get/set_default_env() to env.h

Move these functions to the new header file and rename set_default_env()
to env_set_default() so that it has a consistent env_ prefix.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/board_r.c b/common/board_r.c
index 99b1285..f4193cd 100644
--- a/common/board_r.c
+++ b/common/board_r.c
@@ -448,7 +448,7 @@
 	if (should_load_env())
 		env_relocate();
 	else
-		set_default_env(NULL, 0);
+		env_set_default(NULL, 0);
 #ifdef CONFIG_OF_CONTROL
 	env_set_hex("fdtcontroladdr",
 		    (unsigned long)map_to_sysmem(gd->fdt_blob));