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>

Makefile

@@ -406,8 +406,10 @@ ALL-y += $(obj)u-boot.srec $(obj)u-boot.bin $(obj)System.map
 ALL-$(CONFIG_NAND_U_BOOT) += $(obj)u-boot-nand.bin
 ALL-$(CONFIG_ONENAND_U_BOOT) += $(obj)u-boot-onenand.bin
 ALL-$(CONFIG_SPL) += $(obj)spl/u-boot-spl.bin
-ALL-$(CONFIG_SPL) += $(obj)$(subst ",,$(CONFIG_SPL_TARGET))
 ALL-$(CONFIG_OF_SEPARATE) += $(obj)u-boot.dtb $(obj)u-boot-dtb.bin
+ifneq ($(CONFIG_SPL_TARGET),)
+ALL-$(CONFIG_SPL) += $(obj)$(subst ",,$(CONFIG_SPL_TARGET))
+endif
 
 # enable combined SPL/u-boot/dtb rules for tegra
 ifneq ($(CONFIG_TEGRA),)