irqbalance: add support for uci config

irqbalance: add support for uci config

Add support for uci config file.

Only a global 'enabled" switch has been implemented so far.

* Default config disables irqbalance initially, so that there is
  no change with the previous behaviour (and the possibly existing
  separate launch scripts in people's builds).

Config file and init script can be later extended for irqbalance
options like the oneshot mode or the evaluation interval

(Also change the init file to use tabs)

Signed-off-by: Hannu Nyman <redacted>