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-nova/mk/spec/nova.mk

@@ -0,0 +1,11 @@
+#
+# Specifics for the NOVA kernel API
+#
+
+SPECS += nova
+SPECS += pci ps2 vesa framebuffer
+
+#
+# Linker options that are specific for x86
+#
+LD_TEXT_ADDR ?= 0x01000000

repos/base-nova/mk/spec/nova_x86_32.mk

@@ -0,0 +1,8 @@
+#
+# Specifics for the NOVA kernel API x86 32 bit
+#
+
+SPECS += nova x86_32
+
+include $(call select_from_repositories,mk/spec/x86_32.mk)
+include $(call select_from_repositories,mk/spec/nova.mk)

repos/base-nova/mk/spec/nova_x86_64.mk

@@ -0,0 +1,8 @@
+#
+# Specifics for the NOVA kernel API x86 64 bit
+#
+
+SPECS += nova x86_64
+
+include $(call select_from_repositories,mk/spec/x86_64.mk)
+include $(call select_from_repositories,mk/spec/nova.mk)