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/fdt.c b/drivers/net/fm/fdt.c
index 9918d80..5920fec 100644
--- a/drivers/net/fm/fdt.c
+++ b/drivers/net/fm/fdt.c
@@ -36,7 +36,7 @@
 		return;
 
 	/* If the environment variable is not set, then exit silently */
-	p = getenv("fman_ucode");
+	p = env_get("fman_ucode");
 	if (!p)
 		return;