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/board/buffalo/lsxl/lsxl.c b/board/buffalo/lsxl/lsxl.c
index 0f37345..5b31e05 100644
--- a/board/buffalo/lsxl/lsxl.c
+++ b/board/buffalo/lsxl/lsxl.c
@@ -228,7 +228,7 @@
 static void rescue_mode(void)
 {
 	printf("Entering rescue mode..\n");
-	setenv("bootsource", "rescue");
+	env_set("bootsource", "rescue");
 }
 
 static void check_push_button(void)