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>

 # ARM specific configuration file

 config ARCH_arm

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_SUPPORTS_BOTH_MMU

     select ARCH_DEFAULT_HAS_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     select ARCH_DEFAULT_LE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     select ARCH_SUPPORT_FPU

     help

       The ARM architecture, as defined by:

         http://www.arm.com/