pxeboot: Enforce a 500k limit here

Have a separate PXEBOOTSIZE variable that acts much like LOADERSIZE
variable to limit the size of the loader used for pxeldr. This allows
people to override it independently of LOADERSIZE, which they may need
to set larger for other reasons. Combined with PXEBOOT_DEFAULT_INTERP,
you can build a larger lua loader, while still being able to build pxeldr
with the 4th one, for example.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46214