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/gdsys/a38x/keyprogram.c b/board/gdsys/a38x/keyprogram.c
index a4a6f1c..d75e08b 100644
--- a/board/gdsys/a38x/keyprogram.c
+++ b/board/gdsys/a38x/keyprogram.c
@@ -129,12 +129,12 @@
 	char *hexprog;
 	struct key_program *prog;
 
-	cmd = getenv("loadkeyprogram");
+	cmd = env_get("loadkeyprogram");
 
 	if (!cmd || run_command(cmd, 0))
 		return 1;
 
-	hexprog = getenv("keyprogram");
+	hexprog = env_get("keyprogram");
 
 	if (decode_hexstr(hexprog, &binprog))
 		return 1;