Avoid 'make clean' producing an 'rm' command which has a lot
of duplicate 'hw//*.o' arguments, by using $(sort $(dir ..))
rather than $(dir $(sort ..)) so Make's sort function will
remove the duplicates for us. We can also remove the double
'//' safely because $(dir ..) is guaranteed to return a string
ending in '/'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>