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
Unify 'make prepare' between libports and ports
Add the 'build' directory to '.gitignore'

Issue #1082.
noux: Disable makeinfo for coreutils

Issue #1051
tool_chain: use real patches

Issue #1051.
tool_chain: fix GCC build with makeinfo 5

Issue #1051.
tool_chain: fix GDB build with makeinfo 5

Issue #1051.
build system: Back out on error at libdep stage
Tool for preparing a single 3rd-party port

Issue #1082
Build-system support for ported 3rd-party code

The new 'select_from_ports' function allows a target description file to
query the path to an installed port. All ports are stored in a central
location specified as CONTRIB_DIR. By default, CONTRIB_DIR is defined
as '<genode-dir>/contrib'. Ports of 3rd-party source code are managed
using the tools at '<genode-dir>/tool/ports/'.

Issue #1082
tool/prepare.mk: Switch directories silently
Build-system support for creating host tools
Migrate libports to new ports-management mechanism

Issue #1082
Migrate base-okl4 to new ports management

Issue #1082
Migrate base-nova to new ports management

Issue #1082
Migrate dde_oss to new ports management

Issue #1082
Migrate ports to new ports management (WiP)

Issue #1082
Temporarily disable hash checks

...until the transition to the new ports management is complete.
doc: Description of HOST_TOOLS mechanism
doc: Uniform markup of the build-system manual

Use _italic_ for path names rather than 'verbatim'. Because path names
tend to be quite long, the overly use of verbatim makes paragraphs hard
to read.