simplify compile-time default keyboard map generation

In r296926 the -P <path> option was added to kbdcontrol, which enables
this change for a simplified compile-time default keymap build process.

PR: 193865
Reviewed by: Oliver Pinter
Tested by: Oliver Pinter
MFC After: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5708