Use linker set to declare pre- and post-scheduling tasks

Signed-off-by: Olli Vanhoja <olli.vanhoja@cs.helsinki.fi>
Fixed code formatting and added configurability to some error messages

Signed-off-by: Olli Vanhoja <olli.vanhoja@cs.helsinki.fi>
Use linker sets to define kloggers

- Refactored kerror to use linker sets
- Renamed lastlog logger to kerror_buf
- Added fb logger