Add support for LOADER_RC setting in the pkgfs manifest (defaults to
/loader.rc) to specify a Forth file to read from the pkgfs tarball and
process by Ficl.

This allows for the tarball to do runtime things like load a
platform-specific FDT blob, among other things.

Reviewed by: imp
Approved by: sjg (mentor)
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D8494