env: Rename getenv/_f() to env_get()

We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/net/fec_mxc.c b/drivers/net/fec_mxc.c
index 424c347..797e34f 100644
--- a/drivers/net/fec_mxc.c
+++ b/drivers/net/fec_mxc.c
@@ -1096,7 +1096,7 @@
 			sprintf(mac, "eth%daddr", fec->dev_id);
 		else
 			strcpy(mac, "ethaddr");
-		if (!getenv(mac))
+		if (!env_get(mac))
 			eth_env_set_enetaddr(mac, ethaddr);
 	}
 	return ret;