always build and initialize hwtimer

Eliminates special treatment of the hwtimer module and makes it a
mandatory part of the kernel.
added documentation of configuring the right debug stack size for uart0 module
Merge pull request #776 from OlegHahm/uart0_stack_documentation

added documentation of configuring the right debug stack size for uart0 module
Merge pull request #714 from OlegHahm/hwtimer_integration

core: always build and initialize hwtimer