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/src/lib/target.mk

@@ -0,0 +1,33 @@
+#
+# This is a dummy target description file with the sole purpose of building
+# all libraries.
+#
+TARGET = libs
+
+#
+# Determine all 'lib/mk' sub directories residing within the repositories.
+# Use 'wildcard' to handle the case when a repository does not host any
+# 'lib/mk' sub directory.
+#
+LIB_MK_DIRS := $(wildcard $(addsuffix /lib/mk,$(REPOSITORIES)))
+
+#
+# Scan the 'lib/mk' directories of all repositories for library description
+# files.
+#
+ALL_LIB_MK_FILES := $(notdir $(foreach DIR,$(LIB_MK_DIRS),$(shell find $(DIR) -name "*.mk")))
+
+#
+# Make the pseudo target depend on all libraries, for which an lib.mk file
+# exists. Discard the '.mk' suffix and remove duplicates (via 'sort').
+#
+LIBS = $(sort $(ALL_LIB_MK_FILES:.mk=))
+
+#
+# Among all libraries found above, there may be several libraries with
+# unsatisfied build requirements. Normally, the build system won't attempt to
+# build the target (and its library dependencies) if one or more libraries
+# cannot be built. By enabling 'FORCE_BUILD_LIBS', we let the build system
+# visit all non-invalid libraries even in the presence of invalid libraries.
+#
+FORCE_BUILD_LIBS = yes