env: Rename common functions related to setenv()

We are now using an env_ prefix for environment functions. Rename these
commonly used functions, 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/cmd/setexpr.c b/cmd/setexpr.c
index e9058c4..91a5a9a 100644
--- a/cmd/setexpr.c
+++ b/cmd/setexpr.c
@@ -314,7 +314,7 @@
 
 	/* plain assignment: "setexpr name value" */
 	if (argc == 3) {
-		setenv_hex(argv[1], a);
+		env_set_hex(argv[1], a);
 		return 0;
 	}
 
@@ -370,7 +370,7 @@
 		return 1;
 	}
 
-	setenv_hex(argv[1], value);
+	env_set_hex(argv[1], value);
 
 	return 0;
 }