pxeboot: Allow a different default loader for pxeboot

Sometimes you need / want a different boot loader than loader_lua for
pkeldr. Provide an option to get either the 4th one or the simp one.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46213

(cherry picked from commit f4762e9135d181f5d4f2a9d688ff803ee53ad808)