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/aries/ma5d4evk/ma5d4evk.c b/board/aries/ma5d4evk/ma5d4evk.c
index b9294fc..956c297 100644
--- a/board/aries/ma5d4evk/ma5d4evk.c
+++ b/board/aries/ma5d4evk/ma5d4evk.c
@@ -325,7 +325,7 @@
 
 int board_late_init(void)
 {
-	setenv("bootmode", boot_mode_sf ? "sf" : "emmc");
+	env_set("bootmode", boot_mode_sf ? "sf" : "emmc");
 	return 0;
 }