android: on prepare, clean all intermediates

This was requested to me to prevent no clobbered builds failures
on busybox config changes.

Change-Id: I1bdf2948e0669b1de567381eb809f576e8f1eef5
mktemp: include libc mktemp, safe in busybox

This commit remove the last warning in busybox,
and prevent users to try to use mkstemp which has not
the right behavior.

Change-Id: I5aaae8044769bb1893f2430287d72b3abbee796c