kbuild: clean-up and bug fix of scripts/Makefile.host

Assume we have a Makefile like this:

  hostprogs-y := foo
  foo-cxxobjs := bar/baz.o
  foo-objs := qux/quux.o

In this case, Kbuild creates bar/ directory,
but fails to create qux/ directory.

This commit re-writes directory creation more simply,
fixing that bug.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>