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/samsung/trats/trats.c b/board/samsung/trats/trats.c
index 00059a1..f091338 100644
--- a/board/samsung/trats/trats.c
+++ b/board/samsung/trats/trats.c
@@ -469,7 +469,7 @@
 #endif
 #ifdef CONFIG_S6E8AX0
 	s6e8ax0_init();
-	setenv("lcdinfo", "lcd=s6e8ax0");
+	env_set("lcdinfo", "lcd=s6e8ax0");
 #endif
 }
 #endif