buildtools: store path to buildtools in a variable

Currently, the buildtools are installed relative to ${CT_PREFIX_DIR}.
Change that by introducing ${CT_BUILDTOOLS_DIR}, which is
still set relative to ${CT_PREFIX_DIR}, but which will make it easy
to change in the future.

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

 # Alpha specific configuration file

 config ARCH_alpha

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The Alpha architecture.