build: swap ABS_TOP_BUILDDIR for TOOLS_DIR

build: swap ABS_TOP_BUILDDIR for TOOLS_DIR

All the remaining instances are used to track the tools directory (aka
the local symlinks to kmod). Rename accordingly and include the tools
sub-directory.

Similar to the OVERRIDE_LIBDIR change earlier, this is build system
specific layout, which shouldn't be in the tests.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/86
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>