env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/spl/spl_dfu.c b/common/spl/spl_dfu.c
index e8d0ba1..f5ec0fb 100644
--- a/common/spl/spl_dfu.c
+++ b/common/spl/spl_dfu.c
@@ -46,7 +46,7 @@
 		return -EINVAL;
 	}
 
-	ret = setenv("dfu_alt_info", str_env);
+	ret = env_set("dfu_alt_info", str_env);
 	if (ret) {
 		error("unable to set env variable \"dfu_alt_info\"!\n");
 		return -EINVAL;