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/broadcom/bcm28155_ap/bcm28155_ap.c b/board/broadcom/bcm28155_ap/bcm28155_ap.c
index f5b94f6..8f48ccb 100644
--- a/board/broadcom/bcm28155_ap/bcm28155_ap.c
+++ b/board/broadcom/bcm28155_ap/bcm28155_ap.c
@@ -110,7 +110,7 @@
 int g_dnl_bind_fixup(struct usb_device_descriptor *dev, const char *name)
 {
 	debug("%s\n", __func__);
-	if (!getenv("serial#"))
+	if (!env_get("serial#"))
 		g_dnl_set_serialnumber(CONFIG_USB_SERIALNO);
 	return 0;
 }