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>

net/eth_legacy.c

@@ -261,7 +261,7 @@ int eth_initialize(void)
 		bootstage_error(BOOTSTAGE_ID_NET_ETH_START);
 	} else {
 		struct eth_device *dev = eth_devices;
-		char *ethprime = getenv("ethprime");
+		char *ethprime = env_get("ethprime");
 
 		bootstage_mark(BOOTSTAGE_ID_NET_ETH_INIT);
 		do {