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/common/usb_hub.c b/common/usb_hub.c
index 70bc6e2..86a3477 100644
--- a/common/usb_hub.c
+++ b/common/usb_hub.c
@@ -189,7 +189,7 @@
 	 * but allow this time to be increased via env variable as some
 	 * devices break the spec and require longer warm-up times
 	 */
-	env = getenv("usb_pgood_delay");
+	env = env_get("usb_pgood_delay");
 	if (env)
 		pgood_delay = max(pgood_delay,
 			          (unsigned)simple_strtol(env, NULL, 0));