NCPInstanceBase: Don't allow host sleep while NCP is being initialized
NCPInstanceBase: Don't allow host sleep while NCP is being initialized

This commit changes the `NCPInstanceBase::is_busy()` to ensure that
it returns `true` while NCP is being initialized.

openthread/wpantund,d-bahr/wpantund,abtink/wpantund,abtink/wpantund,openthread/wpantund,abtink/wpantund,aeliot/wpantund,d-bahr/wpantund,aeliot/wpantund,openthread/wpantund,d-bahr/wpantund,aeliot/wpantund,aeliot/wpantund,openthread/wpantund,d-bahr/wpantund,abtink/wpantund