This file system module is somewhat deprecated, but it is still included and maintained because it has some interesting properties that could be useful to a programmer.
The basic concept of fs_builtin is that you pass it a table of ``files'', which really consist of pointers to data blocks and file sizes. It then manages file handles on these files and all of the normal VFS functions work on them.
This used to be used for building files into program images, but that function has been relocated to the much more able fs_romdisk. The interesting property of fs_builtin is that the pointers are arbitrary and point to blocks of data that can be changed on the fly. This could, for example, be used to implement a /proc like filesystem. In practice, it hasn't been used for anything like that.
fs_builtin mounts itself on /bi on the tree.