Create "downstreamkernel_package" just like "downstreamkernel_prepare", to have less redundant code in kernel APKBUILDs

Almost every downstreamkernel's package function has this common block:

package() {
	# kernel.release
	install -D "$builddir/include/config/kernel.release" \
		"$pkgdir/usr/share/kernel/$_flavor/kernel.release"

	# zImage (find the right one)
	cd "$builddir/arch/$_carch/boot"
	_target="$pkgdir/boot/vmlinuz-$_flavor"
	for _zimg in zImage-dtb Image.gz-dtb *zImage Image; do
		[ -e "$_zimg" ] || continue
		msg "zImage found: $_zimg"
		install -Dm644 "$_zimg" "$_target"
		break
	done
	if ! [ -e "$_target" ]; then
		error "Could not find zImage in $PWD!"
		return 1
	fi
}

We are generating it here.

It would be great if we had a downstreamkernel_package function, just like downstreamkernel_prepare, to save this redundant piece at least in new kernels. That makes code reviews quicker, less room for mistakes, less code to understand for the device porters, better maintainability etc 😉

If somebody wants to take a shot at this and needs more information to do it, don't hesitate to ask.

Related: #1570 devicepkg-dev kernel prepare function