kbuild: clean up scripts/Makefile.host

The directory creation can be more simplified by two levels.

[1] Drop $(dir ...)

 $(dir $(f)) always returns non-empty string.

So, $(if $(dir $(f)),$(dir $(f)) is equivalent to $(dir $(f)).

[2] Unroll $(foreach ...) loop

$(dir ...) can take one or more arguments and returns a list of
directories of them.

$(foreach f, $(progs), $(dir $(f))) can be unrolled as $(dir $(progs)).

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