bcmxcp: Define byte for choosing which system test to run in header file
bcmxcp: Use one fuction to decode command execution status in all places

To avoid duplicating the logic which checks the status of command execution at UPS,
add a new function which contains the check, and use that function whenever we
send a command to UPS and get status back from UPS.
bcmxcp: Add test.panel.start instcmd support
bcmxcp: Let decode_instcmd_exec also handle short read from UPS