linux-boundary: Enable CONFIG_FHANDLE on nitrogen6x

This kernel setting is required when using systemd >= 210.  Without
it, no serial devices are enabled at boot time, making serial login
impossible.

Only tested on SabreLite (Nitrogen6x), but the same change should be
applied to all boards.

Signed-off-by: Gary Thomas <gary@mlbassoc.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>