gentoo-overlay/sys-kernel/installkernel-gentoo/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "https://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<maintainer type="project">
		<email>dist-kernel@gentoo.org</email>
		<name>Distribution Kernel Project</name>
	</maintainer>
	<use>
		<flag name="grub" restrict="&lt;sys-kernel/installkernel-gentoo-4">
			Re-generate grub.cfg on each kernel installation
		</flag>
	</use>
</pkgmetadata>