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>

drivers/pci/pci_common.c

@@ -89,7 +89,7 @@ __weak int pci_skip_dev(struct pci_controller *hose, pci_dev_t dev)
 		/*
 		 * Only skip configuration if "pciconfighost" is not set
 		 */
-		if (getenv("pciconfighost") == NULL)
+		if (env_get("pciconfighost") == NULL)
 			return 1;
 #else
 		return 1;