Makefile.target: Update clean command to clean hw/ directory

Makefile.target: Update clean command to clean hw/ directory

Now we create object files in a hierarchy under hw/, so the
'clean' target must also be updated to delete those object files.
Rather than using a manual list of subdirectories which will
easily drift out of date, we just delete all .o and .d files
in the target directory hierarchy.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>