calculate-overlay/sys-boot/grub/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<herd>base-system</herd>
<use>
  <flag name='device-mapper'>Enable support for <pkg>sys-fs/device-mapper</pkg></flag>
  <flag name='multislot'>Allow grub1 and grub2 to be installed simultaneously</flag>
</use>
</pkgmetadata>