gentoo-overlay/sys-freebsd/boot0/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<herd>bsd</herd>
  <use>
    <flag name='tftp'>Enable PXE/TFTP boot support.</flag>
    <flag name='zfs'>Enable booting on ZFS filesystems.</flag>
  </use>
</pkgmetadata>