Replace LOADER_FIREWIRE_SUPPORT variable

Rename LOADER_FIREWIRE_SUPPORT to MK_LOADER_FIREWIRE. Only build
libfirewire when this is "yes". Add note to updating. Fix build script
to build this for x86 so the option doesn't decay. sparc64 supports
ZFS, so also build it MK_ZFS=no.

Sponsored by: Netflix