stand: Stop building in fat, ext2fs, gzip and bzip to BIOS /boot/loader

This saves space to allow pxeboot to work again. Users desiring these
features can turn them on for their custom build. While these are useful
for some specialized applications, they aren't needed to boot the
typical system, and we're low on space.

text    data     bss      dec       hex   filename

Before: 465866 20740 31612 518218 0x7e84a loader_lua.bin
After: 441535 17484 31092 490111 0x77a7f loader_lua.bin

Savings: 28,107 bytes

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D42416

(cherry picked from commit 195a96f0b303345818e09ad1d79dc80122804de8)