make_ext4fs to generate filesystem

The yocto build system has a utility called 'pseudo'.

It uses an LD_PRELOAD library such that anything doing a mknod() libc call is saved to a database instead of invoking the underlying mknod() call, and instead just creates a regular placeholder file.

Then when you run mke2fs, similarly the LD_PRELOAD then hooks stat() such that mke2fs reading the placeholder file actually reads the details from the database instead of the filesystem.

Seems a little hacky at first, but seems to work well enough in practice.