[buteo-syncfw] Introduce DEFINES+=USE_KEEPALIVE

Introduce platform specific code that uses the nemo-keepalive apis.
All existent code is kept intact when this new DEFINES is not used.
This commit also reverts some of changes done by 7674c260812b8656f18e40e58f92efd2e7f8228e
Merge pull request #33 from nemomobile/global-timers

[buteo-syncfw] Introduce DEFINES+=USE_KEEPALIVE