initrd: Add the atkbd kernel module

Since Linux 3.4.74, atkbd gets built as a loadable module.  (Commit
315a93764c1fa8bdbe9ab867d896a0d9478b54ed, "Input: allow deselecting
serio drivers even without CONFIG_EXPERT".)  So because of this we
lost the keyboard in the initrd, which is annoying for LUKS users.

Issue #1405.