Add support for sparse ext4 image creation.

This adds the -s option to the make_ext4fs tool, which now creates
"sparse" filesystem images, which is very useful for the large 32 Gbyte
filesystems we are now building.
This check-in also fixes make_ext4fs to properly create filesystems
larger thatn 4 Gbytes on 64-bit Linux, 32-bit android and Macs.

Change-Id: Ie5838492fcf944f5c875481693c0dbd7013deae4
diff --git a/ext4_utils/ext4_utils.h b/ext4_utils/ext4_utils.h
index 0a2b542..697c8d9 100644
--- a/ext4_utils/ext4_utils.h
+++ b/ext4_utils/ext4_utils.h
@@ -116,7 +116,7 @@
 }
 
 int ext4_bg_has_super_block(int bg);
-void write_ext4_image(const char *filename, int gz);
+void write_ext4_image(const char *filename, int gz, int sparse);
 void ext4_create_fs_aux_info(void);
 void ext4_free_fs_aux_info(void);
 void ext4_fill_in_sb(void);