To build a generic initramfs, you have to install the following software packages:
 * device-mapper
 * cryptsetup-luks
 * rpcbind nfs-utils
 * lvm2
 * iscsi-initiator-utils
 * nbd
 * mdadm
 * net-tools iproute

Generic initramfs'es are huge (usually over 10 megs in size uncompressed), but
should be able to automatically boot any bootable configuration with appropriate
boot flags (root device, network configuration information, etc.)