selftests: lib.mk: define CLEAN macro to allow Makefiles to override clean

Define CLEAN macro to allow Makefiles to override common clean target
in lib.mk. This will help fix the following failures:

warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'

Fixes: 88baa78d ("selftests: remove duplicated all and clean target")

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>