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>

# Blackfin specific configuration file

config ARCH_blackfin

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_DEFAULT_LE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    select ARCH_SUPPORT_FPU

     help

      The Blackfin architecture