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/board/kosagi/novena/novena.c b/board/kosagi/novena/novena.c
index baca722..f0ace03 100644
--- a/board/kosagi/novena/novena.c
+++ b/board/kosagi/novena/novena.c
@@ -240,7 +240,7 @@
 	int ret;
 
 	/* If 'ethaddr' is already set, do nothing. */
-	if (getenv("ethaddr"))
+	if (env_get("ethaddr"))
 		return 0;
 
 	/* EEPROM is at bus 2. */