SHA-512 and constrained devices

[csosto-pk]

As @dstebila pointed out, one concern that comes to mind about using SHA512 even for "level 1" is that on systems with limited space for code base, they might be willing to have one hash function at the right security level (SHA256) but not an additional one at a higher security level (SHA512). Probably not a very big concern, just thought I'd mention it to see what you think.

[csosto-pk]

We should ensure we offer other hash options as this progresses to standardization.