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/highbank/highbank.c b/board/highbank/highbank.c
index 1af2207..e51043a 100644
--- a/board/highbank/highbank.c
+++ b/board/highbank/highbank.c
@@ -82,9 +82,9 @@
 	sprintf(envbuffer, "bootcmd%d", boot_choice);
 	if (getenv(envbuffer)) {
 		sprintf(envbuffer, "run bootcmd%d", boot_choice);
-		setenv("bootcmd", envbuffer);
+		env_set("bootcmd", envbuffer);
 	} else
-		setenv("bootcmd", "");
+		env_set("bootcmd", "");
 
 	return 0;
 }