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/freescale/t4qds/t4240qds.c b/board/freescale/t4qds/t4240qds.c
index 8f9e7e8..2bdd6a3 100644
--- a/board/freescale/t4qds/t4240qds.c
+++ b/board/freescale/t4qds/t4240qds.c
@@ -265,7 +265,7 @@
 	vdd_target = vdd[vid];
 
 	/* check override variable for overriding VDD */
-	vdd_string = getenv("t4240qds_vdd_mv");
+	vdd_string = env_get("t4240qds_vdd_mv");
 	if (vdd_override == 0 && vdd_string &&
 	    !strict_strtoul(vdd_string, 10, &vdd_string_override))
 		vdd_override = vdd_string_override;