libcrypt: add support for SHA256-CRYPT password hashing

libcrypt: add support for SHA256-CRYPT password hashing

This is based on Ulrich Drepper's implementation in GLIBC, but hacked up to work
in uClibc.  The differences from the GLIBC version are as follows:

- b64_from_24bit() has been converted into a macro
- Usage of GLIBC-isms (such as libc_freeres_ptr) have been removed

It is enabled by the UCLIBC_HAS_SHA256_CRYPT_IMPL configuration symbol.  You must
have UCLIBC_HAS_CRYPT_IMPL enabled as well.

Signed-off-by: William Pitcock <nenolod@dereferenced.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>