Allow T to be computed in multiple steps
Allow T to be computed in multiple steps

Previously there were only two states:
- T unallocated
- T allocated and valid

Now there are three:
- T unallocated
- T allocated and in progress
- T allocated and valid

Introduce new bool T_ok to distinguish the last two states.

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