crypto: sha1 - export sha1_update for reuse

Export the update function as crypto_sha1_update() to not have the need
to reimplement the same algorithm for each SHA-1 implementation. This
way the generic SHA-1 implementation can be used as fallback for other
implementations that fail to run under certain circumstances, like the
need for an FPU context while executing in IRQ context.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>