kbuild: handle the dependency of multi-objs hostprogs appropriately

Assume we have a Makefile like:

hostprogs-y := foo bar
foo-objs := foo1.o foo2.o
bar-objs := bar1.o bar2.o

Without this commit, the host program foo depends on all of
foo1.o foo2.o bar1.o bar2.o.

This commit allows to handle the dependency of each host program
separately.

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