build: Fix make errors generated when building 'distclean'

It was noticed that when `make distclean' is run, the make process
terminates with error reporting something like:

rm: cannot remove '/tmp/foobar/': Is a directory
make: *** [clobber] Error 1

The problem is that the list of files targeted for removal includes a
directory in case CONFIG_SPL_TARGET is not set.

The fix has been tested as follows:

 Ran several times the following sequence of commands:

     CROSS_COMPILE=/usr/bin/arm-linux-gnueabi- make O=/tmp/foobar smdk5250_config
     CROSS_COMPILE=/usr/bin/arm-linux-gnueabi- make O=/tmp/foobar distclean

 it did not cause an error, it used to before this change.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>