Avoid having an explicit list of directories in the 'clean'
target by using 'find' to remove all .o and .d files instead.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>