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/net/fm/b4860.c b/drivers/net/fm/b4860.c
index 5aeeb87..3245bee 100644
--- a/drivers/net/fm/b4860.c
+++ b/drivers/net/fm/b4860.c
@@ -97,7 +97,7 @@
 			 * Extract hwconfig from environment since environment
 			 * is not setup yet
 			 */
-			getenv_f("hwconfig", buffer, sizeof(buffer));
+			env_get_f("hwconfig", buffer, sizeof(buffer));
 			buf = buffer;
 
 			/* check if XFI interface enable in hwconfig for 10g */