cmake: Try and improve cleaning of builds when B==S

cmake: Try and improve cleaning of builds when B==S

Currently if B==S for a cmake recipe, the build will not reconfigure. This patch adds
code to remove the generated cmake files, meaning cmake will then be forced to regenerate
them. This forces cmake to see configuration changes it may not otherwise see.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>