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.