Makefile: fix "make clean" for cross-compile

Makefile: fix "make clean" for cross-compile

When cross-compiling (make CROSS=mipsel-linux-), I get this error:

rm -rf /home/norris/git/mtd-utils/mipsel-linux
find /home/norris/git/mtd-utils/mipsel-linux/ -xdev \
		'(' -name '*.[ao]' -o -name '.*.c.dep' ')' \
		-exec rm -f {} +
find: `/home/norris/git/mtd-utils/mipsel-linux/': No such file or directory
make: *** [clean] Error 1

Now, for cross-compiling, we've already deleted the $(BUILDDIR), so we
should check this before cleaning individual objects with "find."

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>