Add generic wrappers for performing message digest operations.

Add generic wrappers for performing message digest operations.

A DigestContext structure now wraps state for all the SHA* algorithms. Replaced the old SHA*_file functions to use these new wrappers. Also moved the digest utility functions from tests/ to crypto/. 

BUG=670
TEST=Modified existing tests to use the new wrappers and they pass.

[Chris Masone, 2010-02-05 22:22:39]

lgtm with that comment.

http://codereview.chromium.org/579007/diff/1/3
File src/platform/vboot_reference/crypto/sha_utility.c (right):

http://codereview.chromium.org/579007/diff/1/3#newcode111
src/platform/vboot_reference/crypto/sha_utility.c:111: uint8_t*
DigestBuf(uint8_t* buf, int len, int sig_algorithm) {
some comments explaining what's going on in this fcn might help those whose C-fu
is a bit weak.  Function pointers can be mindbending.