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/etc/README

@@ -0,0 +1,16 @@
+This directory contains default configuration files that are used
+by the '<role>.mk' files in the 'mk/' directory. By
+convention, configuration files are first read from here
+followed by a corresponding config file in '<builddir>/etc/'.
+
+
+Convention
+~~~~~~~~~~
+
+We include config files directly into makefiles. So the basic
+makefile syntax applies here, too.
+
+Config files should
+* Have '.conf' as filename extension.
+* Use only assignments but provide no rules or other 'make'-magic.
+* Not include other files!

repos/base/etc/tools.conf

@@ -0,0 +1,57 @@
+#
+# The following options let you define non-default tools to use
+#
+# CUSTOM_LD is only used for the progressive linking of libraries.
+# It is not used for linking the final target.
+#
+#CUSTOM_CC      = gcc
+#CUSTOM_CXX     = g++
+#CUSTOM_AS      = as
+#CUSTOM_LD      = ld
+
+#
+# For using a cross-compile tool chain, the names of all
+# binutils and compilers are typically prefixed by the
+# target platform. Instead of defining CUSTOM_* variables
+# individually for each tool, the prefix can be defined
+# via the following variable.
+#
+ifeq ($(filter-out $(SPECS),x86),)
+CROSS_DEV_PREFIX ?= /usr/local/genode-gcc/bin/genode-x86-
+endif
+ifeq ($(filter-out $(SPECS),arm),)
+CROSS_DEV_PREFIX ?= /usr/local/genode-gcc/bin/genode-arm-
+endif
+
+#
+# We use libsupc++ from g++ version 3 because
+# this version does not use thread-local storage
+# via the gs register. This is an interim solution.
+#
+#CUSTOM_CXX_LIB = g++-3.4
+
+#
+# The default optimization level used for compiling is -O2.
+# By defining the variable CC_OLEVEL, you can override this
+# default value, for example to optimize your binaries for size.
+#
+#CC_OLEVEL = -Os
+
+#
+# If CC_OPT should be extended please use concatenation syntax like:
+#
+#CC_OPT += -ffunction-sections -fdata-sections
+
+#
+# If CXX_LINK_OPT (linker options given to CXX)  should be extended please use
+# concatenation syntax like:
+#
+#CXX_LINK_OPT += -Wl,-gc-sections
+
+#
+# On non-GNU systems, you may direct the build system to use GNU-
+# specific tools.
+#
+#TAC              ?= /opt/gnu/bin/tac
+#GNU_FIND         ?= /opt/gnu/bin/find
+#GNU_XARGS        ?= /opt/gnu/bin/xargs