The actual stable genkernel say in "--help" following: --luks Include LUKS support --> 'emerge cryptsetup-luks' with USE=-dynamic Thats wrong. There is no such ebuild.
should be fixed by: http://gitweb.gentoo.org/proj/genkernel.git/commit/?id=8434722d7bfcb4b6d7d6dc7f6f5f6330bbe9289a