Give loaders more control over the Forth initialization process. In
particular, allow loaders to define the name of the RC script the
interpreter needs to use. Use this new-found control to have the
PXE loader (when compiled with TFTP support and not NFS support)
read from ${bootfile}.4th, where ${bootfile} is the name of the
file fetched by the PXE firmware.

The normal startup process involves reading the following files:
1.  /boot/boot.4th
2.  /boot/loader.rc or alternatively /boot/boot.conf

When these come from a FreeBSD-defined file system, this is all
good. But when we boot over the network, subdirectories and fixed
file names are often painful to administrators and there's really
no way for them to change the behaviour of the loader.

Obtained from:	Juniper Networks, Inc.