env: Rename common functions related to setenv()

We are now using an env_ prefix for environment functions. Rename these
commonly used functions, for consistency. Also add function comments in
common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/lzmadec.c

@@ -42,7 +42,7 @@ static int do_lzmadec(cmd_tbl_t *cmdtp, int flag, int argc, char *const argv[])
 		return 1;
 	printf("Uncompressed size: %ld = %#lX\n", (ulong)src_len,
 	       (ulong)src_len);
-	setenv_hex("filesize", src_len);
+	env_set_hex("filesize", src_len);
 
 	return 0;
 }