arm64: Use full path in KBUILD_IMAGE definition

The KBUILD_IMAGE variable is used by the rpm and deb-pkg targets, which
expect it to point to the image file in the build directory. The
builddeb script has a workaround for architectures which only provide
the basename, but let's provide a clean interface for packaging tools.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Michal Marek <mmarek@suse.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>