drivers: reverting Kconfig/Makefile changes for slip

The slip driver revert (3e63a74) was incomplete, missing the Kconfig and
Makefile changes, causing 'make clean' to fail.

Change-Id: I9d944148e6be3756b62d2371a5ab5528365e1ec1
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
2 files changed