Introduce 'spec' subdirectories to outline aspects

Instead of holding SPEC-variable dependent files and directories inline
within the repository structure, move them into 'spec' subdirectories
at the corresponding levels, e.g.:

  repos/base/include/spec
  repos/base/mk/spec
  repos/base/lib/mk/spec
  repos/base/src/core/spec
  ...

Moreover, this commit removes the 'platform' directories. That term was
used in an overloaded sense. All SPEC-relative 'platform' directories are
now named 'spec'. Other files, like for instance those related to the
kernel/architecture specific startup library, where moved from 'platform'
directories to explicit, more meaningful places like e.g.: 'src/lib/startup'.

Fix #1673

repos/base-okl4/lib/mk/base-common.mk

@@ -27,6 +27,6 @@ INC_DIR +=  $(REP_DIR)/src/base/lock
 INC_DIR += $(BASE_DIR)/src/base/lock
 INC_DIR += $(BASE_DIR)/src/base/thread
 
-vpath cap_copy.cc $(BASE_DIR)/src/platform
+vpath cap_copy.cc $(BASE_DIR)/src/lib/startup
 vpath %.cc        $(REP_DIR)/src/base
 vpath %.cc        $(BASE_DIR)/src/base