common: Move gzip functions into a new gzip header

As part of the effort to remove things from common.h, create a new header
for the gzip functions. Move the function declarations to it and add
missing documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>

cmd/config.c

@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <gzip.h>
 #include <malloc.h>
 
 #include "config_data_gz.h"