This reverts commit 67871a59.

Since that commit, out-of-tree cleaning emits lots of warnings.

$ make O=foo clean
make[1]: Entering directory `/home/masahiro/workspace/u-boot/foo'
../dts/../arch/arm/dts/Makefile:209: warning: overriding commands for target `dtbs'
../dts/../arch/arc/dts/Makefile:15: warning: ignoring old commands for target `dtbs'
../dts/../arch/microblaze/dts/Makefile:13: warning: overriding commands for target `dtbs'
../dts/../arch/arm/dts/Makefile:209: warning: ignoring old commands for target `dtbs'
../dts/../arch/mips/dts/Makefile:14: warning: overriding commands for target `dtbs'
../dts/../arch/microblaze/dts/Makefile:13: warning: ignoring old commands for target `dtbs'
../dts/../arch/nios2/dts/Makefile:13: warning: overriding commands for target `dtbs'
../dts/../arch/mips/dts/Makefile:14: warning: ignoring old commands for target `dtbs'
../dts/../arch/powerpc/dts/Makefile:13: warning: overriding commands for target `dtbs'
../dts/../arch/nios2/dts/Makefile:13: warning: ignoring old commands for target `dtbs'
../dts/../arch/sandbox/dts/Makefile:14: warning: overriding commands for target `dtbs'
../dts/../arch/powerpc/dts/Makefile:13: warning: ignoring old commands for target `dtbs'
../dts/../arch/x86/dts/Makefile:22: warning: overriding commands for target `dtbs'
../dts/../arch/sandbox/dts/Makefile:14: warning: ignoring old commands for target `dtbs'
make[1]: Leaving directory `/home/masahiro/workspace/u-boot/foo'

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>