fix error message for validate task

[GwendalLaurent]

This PR fixes reintroduce the -d option for the validation task

The new help message is now the following one:

Validate an update deployed on a specific device

Example: rebar3 grisp-io validate -d 1337

Usage: rebar3 grisp-io validate [-d <device>]

  -d, --device  Specify the serial number of the device

The test suite has been updated and the "skip" is now only applied to the testcase that is supposed to run successfully in the waiting of a fix.

The README has also been updated accordingly