timer: Build platform-specific parts as lib

By using the build system's library-selection mechanism instead of many
timer targets with different 'REQUIRES' declarations, this patch reduces
the noise of the build system. For all platforms, the target at
'os/src/drivers/timer' is built. The target, in turn, depends on a
'timer' library, which is platform-specific. The various library
description files are located under 'os/lib/mk/<platform>'. The common
bits are contained in 'os/lib/mk/timer.inc'.