Move the icky call to step() from aoe.c to ata.c; this takes it at
least one step further away from where it really doesn't belong.

Unfortunately, AoE has the ugly aoe_discover() mechanism which means
that we still have a step() loop in aoe.c for now; this needs to be
replaced at some future point.

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

iSCSI hack.

layer asynchronous, so keeping the sync/async boundary within aoedev.c
seems cleanest for now.

async_wait(), though we may wish to move this higher up the stack, and
consider making the block device model asynchronous.  (There is only a
marginal cost for synchronous devices, since they can simply call
async_done() before returning; async_wait() will work seamlessly in this
situation).

Removed data_in_len and data_out_len from ata_command structure; the
lengths are implied by the sector count and the presence of the data_in
or data_out pointers.

Changed AoE code to use subcommands by default, and made aoe_issue()
nonblocking (with completion via async_wait()).

Increase code simplicity at the expense of around 64 bytes.

enough to support both real IDE chipsets and AoE.