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
native: removed unreachable code
Merge pull request #779 from mehlis/native-unreachable-code

native: removed unreachable code
build id shell command only for config module

closes #777
ifdef shell _heap_handler for corresponding module
Merge pull request #781 from LudwigOrtmann/sc_id

build id shell command only for config 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