Ack timeout must be at least 2 seconds
Ack timeout must be at least 2 seconds

While calculating ack timeout, we were ending up getting a random value
which may become less than 2 seconds. This is not allowed as per v1.0.2
specification.

To fix the issue we now take the random number from 0 to 2000 ms and
then add that to the fixed 2000 ms ack timeout value, guaranteeing a
value at least equal to 2000 ms.

andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed