[crypto] Add concept of cipher alignment size

The GCM cipher mode of operation (in common with other counter-based
modes of operation) has a notion of blocksize that does not neatly
fall into our current abstraction: it does operate in 16-byte blocks
but allows for an arbitrary overall data length (i.e. the final block
may be incomplete).

Model this by adding a concept of alignment size.  Each call to
encrypt() or decrypt() must begin at a multiple of the alignment size
from the start of the data stream.  This allows us to model GCM by
using a block size of 1 byte and an alignment size of 16 bytes.

As a side benefit, this same concept allows us to neatly model the
fact that raw AES can encrypt only a single 16-byte block, by
specifying an alignment size of zero on this cipher.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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