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>

board/technexion/twister/twister.c

@@ -94,7 +94,7 @@ int misc_init_r(void)
 
 	omap_die_id_display();
 
-	eth_addr = getenv("ethaddr");
+	eth_addr = env_get("ethaddr");
 	if (eth_addr)
 		return 0;