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

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>