Fix missing offset in TBStore read from flash
Fix missing offset in TBStore read from flash

This could cause incomplete data retrieval and mismatch when reading
data in more than one chunk, because every chunk would be read to the
same location at the beginning of the output buffer.

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