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/cmd/gpt.c b/cmd/gpt.c
index 6f08f61..fbfbe7b 100644
--- a/cmd/gpt.c
+++ b/cmd/gpt.c
@@ -60,7 +60,7 @@
 #ifdef CONFIG_RANDOM_UUID
 		debug("%s unset. ", str);
 		gen_rand_uuid_str(uuid_str, UUID_STR_FORMAT_GUID);
-		setenv(s, uuid_str);
+		env_set(s, uuid_str);
 
 		e = getenv(s);
 		if (e) {
@@ -626,7 +626,7 @@
 		return CMD_RET_FAILURE;
 
 	if (namestr)
-		setenv(namestr, disk_guid);
+		env_set(namestr, disk_guid);
 	else
 		printf("%s\n", disk_guid);