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/bosch/shc/board.c b/board/bosch/shc/board.c
index 38577f3..2c830a2 100644
--- a/board/bosch/shc/board.c
+++ b/board/bosch/shc/board.c
@@ -251,7 +251,7 @@
 
 	if (value == 0) {
 		printf("front button activated !\n");
-		setenv("harakiri", "1");
+		env_set("harakiri", "1");
 	}
 }