Avoid use of REP_DIR in *.mk files to simplify the use of these files as
templates for a board hosted in a separate repository.
Use REP_INC_DIR for searching headers, thereby considering headers
hosted in an external repository.
Issue #3168
Put the initialization of the cpu cores, setup of page-tables, enabling of
MMU and caches into a separate component that is only used to bootstrap
the kernel resp. core.
Ref #2092