nanobsd: Add missing options to usage()

-B suppress installs (both kernel and world)
-I build disk image from existing build/install
-W suppress installworld

PR: 277269
Reviewed by: imp, emaste
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47475

(cherry picked from commit 999f288a0eeb230d3655da94c1186ca03c0cc404)