Introducing hostprogs-y allows a user to use the typical Kbuild
pattern in a Kbuild file:
hostprogs-$(CONFIG_KALLSYMS) += ...

And then during cleaning the referenced file are still deleted.
Deprecate the old host-progs assignment but kept the functionlity.

External modules will continue to use host-progs for a while - drawback is
that they now see a warning.
Workaround - just assign both variables:
hostprogs-y := foo
host-progs  := $(hostprogs-y)

All in-kernel users will be converted in next patch.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>