Fix clean when all platforms specified

When running:
 ./build-all.sh -p all clean
Ensure that "clean" is passed down when looping over each platform. Previously,
instead it would loop over all platforms building and packaging.

Change-Id: Ic03f292d6164943105de9bf39f15c8e7e7e344eb
Signed-off-by: Dean Birch <dean.birch@arm.com>