kbuild: process mixture of clean/build targets one by one

Support parallel building of clean, config, and build targets in a
single command.

For example,

  make -j<N> clean all

or

  make -j<N> mrproper defconfig all

They should be handled one by one.

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