Move repositories to 'repos/' subdirectory

This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.

Issue #1082

repos/base-codezero/mk/spec-codezero.mk

@@ -0,0 +1,51 @@
+#
+# Specifics for the Codezero kernel API
+#
+
+#
+# Read default and builddir-specific config files
+#
+# In these config files, we expect to find the definition of CODEZERO_DIR
+#
+-include $(call select_from_repositories,etc/codezero.conf)
+-include $(BUILD_BASE_DIR)/etc/codezero.conf
+
+ifeq ($(CODEZERO_DIR),)
+$(error Could not find the definition of CODEZERO_DIR in etc/codezero.conf)
+endif
+
+#
+# Convert path to absolute directory
+#
+absdir = $(shell readlink -f $(1))
+
+#
+# Headers generated within the build directory
+# (see 'lib/mk/platform.mk')
+#
+INC_DIR += $(BUILD_BASE_DIR)/include
+
+#
+# Codezero headers
+#
+CODEZERO_ABS_DIR = $(call absdir,$(CODEZERO_DIR))
+
+INC_DIR += $(CODEZERO_ABS_DIR)/include
+INC_DIR += $(CODEZERO_ABS_DIR)/conts/userlibs/libl4/include
+INC_DIR += $(CODEZERO_ABS_DIR)/conts/userlibs/libdev/uart/include
+
+#
+# Allow programs to test for the Codezero kernel
+#
+# This is needed by the 'pl050/irq_handler.h' to handle the interrupt semantics
+# of Codezero.
+#
+CC_OPT += -D__CODEZERO__
+
+#
+# Clean rules for removing the side effects of building the platform
+#
+clean_includes:
+	$(VERBOSE)rm -rf $(BUILD_BASE_DIR)/include
+
+cleanall: clean_includes