Import pkgtools/bootstrap-mk-files, which is the collection of bmake *.mk

Import pkgtools/bootstrap-mk-files, which is the collection of bmake *.mk
files from the bootstrap kit.  This package is used during bootstrap to
properly register the installed *.mk files.

The bootstrap bmake(1) utility uses some customised .mk files in order
to pre-define certain definitions and targets, which guide the build
process.  This package provides those *.mk files for the bmake(1)
utility.