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/disk/part.c b/disk/part.c
index d76aa90..c67fdac 100644
--- a/disk/part.c
+++ b/disk/part.c
@@ -455,7 +455,7 @@
 	/* If no dev_part_str, use bootdevice environment variable */
 	if (!dev_part_str || !strlen(dev_part_str) ||
 	    !strcmp(dev_part_str, "-"))
-		dev_part_str = getenv("bootdevice");
+		dev_part_str = env_get("bootdevice");
 
 	/* If still no dev_part_str, it's an error */
 	if (!dev_part_str) {