parisc: Fix KBUILD_IMAGE for self-extracting kernel

Default KBUILD_IMAGE to $(boot)/bzImage if a self-extracting
(CONFIG_PARISC_SELF_EXTRACT=y) kernel is to be built.
This fixes the bindeb-pkg make target.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # v4.14+