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/drivers/input/input.c b/drivers/input/input.c
index 011667f..26da3a9 100644
--- a/drivers/input/input.c
+++ b/drivers/input/input.c
@@ -652,7 +652,7 @@
 	error = stdio_register(dev);
 
 	/* check if this is the standard input device */
-	if (!error && strcmp(getenv("stdin"), dev->name) == 0) {
+	if (!error && strcmp(env_get("stdin"), dev->name) == 0) {
 		/* reassign the console */
 		if (OVERWRITE_CONSOLE ||
 				console_assign(stdin, dev->name))