Make SHA-2 implementation smaller
Make SHA-2 implementation smaller

Adjust the size/performance trade-off:
* Reduces size of sha256_process() from 7.4KB to 2KB on ARMv7-M
* Reduces performance by less than 14% on Cortex-M4
* Seems to even improve performance on my Core i7

Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls