For that, make CT_BUILD_TOP_DIR a non-settable config option (so that it is
recursively expanded with CT_HOST/CT_TARGET). Use a common prefix, with
same default as for regular sample build.
Use showConfig.sh to determine host toolchain path (for canadian crosses)
and build directory to be removed.
Remove LIBC_SYSROOT_ARG (unused).
Signed-off-by: Alexey Neyman <stilor@att.net>