buildtools: move to working directory

There is absolutely *no* reason for the buildtools (wrappers to gcc, g++,
as, ld... for the local machine) to be in the toolchain directory. Moreover,
they are removed after the build completes.

Move them out of the toolchain directory, and into the build directory (but
yet the part specific to the current toolchain). This means we no longer
need to explicitly remove them either, BTW, but we need to save/restore them
for the restart feature.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# MIPS specific config options

config ARCH_mips

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64 if EXPERIMENTAL

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORTS_BOTH_ENDIAN

    select ARCH_DEFAULT_BE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_TUNE

    help

      The MIPS architecture, as defined by:

        http://www.mips.com/