images/gentoo: installkernel >=20 updates

[juippis]

• add "grub" use flag with openrc (I forgot this last time),
• "systemd-kernel-install" was settled with "systemd-boot" instead on systemd.

NOTE: I couldn't boot the systemd variant, but openrc fully works. I'm gonna need some time to study systemd-boot as I've never dealt with it before. However the systemd-boot use flag looks final, so this should still be merged if systemd-boot is to be used in future with systemd vm variants.

[juippis]

Just throwing my observation about systemd-boot here - not sure if it's relevant or not: The /boot files are installed on the "EFI" partition (/dev/sda1) while with openrc they are installed on rootfs (/dev/sda2). No separate /boot is mounted on either case.

[juippis]

Force-pushed an update. Adding
echo "root=/dev/sda2" >> /etc/kernel/cmdline
makes the systemd vm work for me locally - not sure if /dev/sda2 is something that'd only work here, but I think that's a hint of sort what causes the issue. Please take it from here :)