[crypto] Allow for AES hardware acceleration

Allow architectures to detect support for AES hardware acceleration at
runtime and to replace the AES algorithm's encrypt() and decrypt()
method pointers with hardware accelerated implementations.

Extend the automated tests to run the AES tests twice: once with
hardware acceleration explicitly disabled (to test the unaccelerated
software implementation) and once with acceleration re-enabled.  Skip
the second test if no hardware acceleration is available: this avoids
unnecessarily repeating the test of the unaccelerated implementation,
and allows a non-zero test count for "aes-hw" to indicate that the
hardware acceleration was tested.  For example:

On a system that supports AES hardware acceleration:

   OK: "aes" 120 tests passed
   OK: "aes-hw" 120 tests passed

On a system that does not support AES hardware acceleration:

   OK: "aes" 120 tests passed
   OK: "aes-hw" 0 tests passed

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