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
* Remove SPEC declarations from mk/spec
* Remove all board-specific REQUIRE declaratiions left
* Replace [have_spec <board>] run-script declarations with have_board where necessary
* Remove addition of BOARD variable to SPECS in toplevel Makefile
* Move board-specific directories in base-hw out of specs