kbuild: Use HOST*FLAGS options from the command line

Now that we have the rename in place, reuse the HOST*FLAGS options as
something that can be set from the command line and included with the
rest of the flags.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>