MAC operations now supports NULL arguments

MAC algorithms support NULL arguments and zero length strings.

Note that the fix consists in a change of API in the internal crypto
interface. This change make hash_ops and mac_ops look the same in terms of
update and final step

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Docs: Add platform specific details

Update README.md with information that tells how to build for the
different software and hardware combinations.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>