To create the /tmp directory on a RAM-disk, you can use the following in your buildfile:

[type=link] /tmp = /dev/shmem

This will establish /tmp as a symbolic link in the process manager's pathname table to the /dev/shmem directory. Since the /dev/shmem directory is really the place where shared memory objects are stored, this effectively lets you create files on a RAM-disk — files created are, in reality, shared memory objects living in RAM.

Note that the line containing the link attribute (the [type=link] line) should be placed outside of the script file or boot file — after all, you're telling mkifs that it should create a file that just happens to be a link rather than a "real" file.