flashtool: allow failure to read ELFLoader ACK byte
flashtool: allow failure to read ELFLoader ACK byte

For some unknown reason not all setups can reliably read the dummy
ACK byte transmitted Valiant -> workstation after TX
workstation -> Valiant. Flashing succeeds in most such cases anyway
so replace the hard failure with a warning.

Change-Id: Iae9c534fccf54fab98d94d8a9ab6a734bb0cbd1a
GitOrigin-RevId: 6c16b17f8598028e168efc8f9c8aeecb0d062c72

google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro