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
fixup Move (tool: fix BASE_DIR in build.conf)
fixup Move (fix tool path in Makefiles)
fixup Move (fix tool path in gcc and gdb ports)
Unify 'make prepare' between libports and ports
hw: make Processor_client::_processor protected

ref #1126
hw: turn Cortex_a9::Processor_driver into a class

ref #1126
hw & rpi: provide Pic::MAX_INTERRUPT_ID

ref #1126
hw & cortex_a9: do lazy FPU-context switch

ref #1126
base & cortex_a9: enable FPU

ref #1126
base: test for the pseudo-parallel use of an FPU

fix #1126
hw: move double list to extra header

ref #1139
hw: fix type casting in Double_list

ref #1139
hw: implement Double_list::for_each

ref #1139
hw: TLB flushes via Kernel::Processor_broadcast

ref #1139
hw: make boot_modules.s generic

ref #1139
hw: make build_boot_image in run/env generic

ref #1139
base: extra section for binaries of boot modules

This enables us to map the binaries of the boot modules on demand at least in
base-hw.

ref #1139
hw: map binaries of boot modules on demand

fix #1139
hw: align kernel stack appropriately

ref #1144