kbuild: create output directory for hostprogs with O=.. build

hostprogs-y only supported creating output directory for the final
program. Extend this to also cover the situation where a .o
file (used when host program is made from compositie objects) is
locate in another directory.
First user of this is the built-in lxdialog that.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>