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>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =