mbedtls: Disable MBEDTLS_SHA256_SMALLER implementation

mbedtls: Disable MBEDTLS_SHA256_SMALLER implementation

Disable MBEDTLS_SHA256_SMALLER implementation, not enabled by default in
upstream and reduces performance by quite a bit.

Source: include/mbedtls/config.h

Enable an implementation of SHA-256 that has lower ROM footprint but also
lower performance.

The default implementation is meant to be a reasonnable compromise between
performance and size. This version optimizes more aggressively for size at
the expense of performance. Eg on Cortex-M4 it reduces the size of
mbedtls_sha256_process() from ~2KB to ~0.5KB for a performance hit of
about 30%.

The size of mbedtls increased a little bit:
ipkg for mips_24kc before:
164.382 Bytes
ipkg for mips_24kc after:
166.240 Bytes

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>