Add Dist-kernel kconfig support

[immolo]

Based on the orginal patch by oldfashionedcow to add the abilty for a user to set a custom kconfig for their livecd to be moved to /etc/kernel/config.d/ before sys-kernel/gentoo-kernel is emerged.

Syntax is the same as the one used for Genkernel kconfigs.

Closes: https://bugs.gentoo.org/936676