kbuild: pass $(MAKECMDGOALS) to sub-make as is

Manipulating $(MAKECMDGOALS) for sub-make seems odd to me.

[1] 'make O=foo sub-make' is turned into 'make O=foo', which builds
the default targets. It would make sense to terminate the build with:

  *** No rule to make target 'sub-make'.  Stop.

[2] 'make O=foo defconfig _all' is turned into 'make O=foo defconfig',
which changes the behavior.

Let's pass $(MAKECMDGOALS) as is.

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