kbuild: define DO_DEPS_ONLY for u-boot.cfg to fix build error

Since 741e58e0fc8e (Create a .cfg file containing the CONFIG options
used to build), all the Blackfin boards fail to build if the parallel
(-j) option is passed.

  $ make -s bf506f-ezkit_defconfig
  #
  # configuration written to .config
  #
  $ make -j8 CROSS_COMPILE=bfin-elf-
  scripts/kconfig/conf --silentoldconfig Kconfig
    CHK     include/config.h
    UPD     include/config.h
    GEN     include/autoconf.mk
    GEN     include/autoconf.mk.dep
    CHK     include/config/uboot.release
    CHK     include/generated/timestamp_autogenerated.h
    UPD     include/generated/timestamp_autogenerated.h
    CFG     u-boot.cfg
  include/asm-offsets.h:3:43: fatal error:
  generated/generic-asm-offsets.h: No such file or directory
  compilation terminated.
  make: *** [u-boot.cfg] Error 1

When parsing header files for defined CONFIG options, DO_DEPS_ONLY
must be defined to exclude generated headers that might not have
been available yet.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index ea67137..fd572f4 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -167,7 +167,7 @@
 
 quiet_cmd_cpp_cfg = CFG     $@
 cmd_cpp_cfg = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
-		-D__ASSEMBLY__ -x assembler-with-cpp -P -dM -E -o $@ $<
+	-DDO_DEPS_ONLY -D__ASSEMBLY__ -x assembler-with-cpp -P -dM -E -o $@ $<
 
 $(obj)/$(SPL_BIN).cfg:	include/config.h
 	$(call if_changed,cpp_cfg)