[feature][mbed] Set `XI_USER_AGENT` -
[feature][mbed] Set `XI_USER_AGENT` -

Simply append it to `xi_consts.h`, as there is no global constant which we can rely on.
The issue had been raised with mbed team and hopefully `__MBED__` will soon appear!

foss-for-synopsys-dwc-arc-processors/libxively,foss-for-synopsys-dwc-arc-processors/libxively