Merge from CheriBSD:

Make the sysinit tool a build tool rather than building in with
/usr/bin/cc and running it from OBJDIR.  (It will be moved to usr.bin
once a manpage is written and a few style cleanups are done.)

Split the makefile bits for Hans' kernel shim layer into their own
includable kshim.mk.

Move USB support into a .mk file so loaders can include it.