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/CZ.NIC/turris_omnia/turris_omnia.c b/board/CZ.NIC/turris_omnia/turris_omnia.c
index a427509..f63e3b6 100644
--- a/board/CZ.NIC/turris_omnia/turris_omnia.c
+++ b/board/CZ.NIC/turris_omnia/turris_omnia.c
@@ -285,7 +285,7 @@
 		puts("EEPROM regdomain read failed.\n");
 
 	printf("Regdomain set to %s\n", rd);
-	return setenv("regdomain", rd);
+	return env_set("regdomain", rd);
 }
 #endif