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/boundary/nitrogen6x/nitrogen6x.c b/board/boundary/nitrogen6x/nitrogen6x.c
index 17fd6f5..c04926c 100644
--- a/board/boundary/nitrogen6x/nitrogen6x.c
+++ b/board/boundary/nitrogen6x/nitrogen6x.c
@@ -954,7 +954,7 @@
 {
 	char envvalue[ARRAY_SIZE(buttons)+1];
 	int numpressed = read_keys(envvalue);
-	setenv("keybd", envvalue);
+	env_set("keybd", envvalue);
 	return numpressed == 0;
 }
 
@@ -995,7 +995,7 @@
 			sprintf(cmd_name, "%s%c", kbd_command_prefix, *suffix);
 			cmd = getenv(cmd_name);
 			if (cmd) {
-				setenv("preboot", cmd);
+				env_set("preboot", cmd);
 				return;
 			}
 		}