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/renesas/ecovec/ecovec.c b/board/renesas/ecovec/ecovec.c
index 28b557a..e4bb440 100644
--- a/board/renesas/ecovec/ecovec.c
+++ b/board/renesas/ecovec/ecovec.c
@@ -55,7 +55,7 @@
 	/* Set MAC address */
 	sprintf(env_mac, "%02X:%02X:%02X:%02X:%02X:%02X",
 		mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
-	setenv("ethaddr", env_mac);
+	env_set("ethaddr", env_mac);
 
 	debug_led(0x0F);