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/samtec/vining_2000/vining_2000.c b/board/samtec/vining_2000/vining_2000.c
index 5c8b436..d15f2a9 100644
--- a/board/samtec/vining_2000/vining_2000.c
+++ b/board/samtec/vining_2000/vining_2000.c
@@ -413,11 +413,11 @@
 		return ret;
 
 	if (val >= VAL_UPPER)
-		setenv("pin_state", "connected");
+		env_set("pin_state", "connected");
 	else if (val < VAL_UPPER && val > VAL_LOWER)
-		setenv("pin_state", "open");
+		env_set("pin_state", "open");
 	else
-		setenv("pin_state", "button");
+		env_set("pin_state", "button");
 
 	return ret;
 }