Minor modifications to make build-scripts robust

* kvm-unit-test: add config step before "make distclean"
to align with upstream Makefile of kvm-unit-test

* buildroot: add steps to remove the temporary directories
that may be left out in case of a previous build has been
interrupted midway before completion

Signed-off-by: Raghuram Thammiraju <raghuram.thammiraju@arm.com>
Change-Id: Ie69e3de8779d427e546f3df4318e6da2a403a29a