linux-postmarketos-allwinner: Use postmarketos-installkernel, get rid of downstreamkernel_*

The downstreamkernel_* functions assume the presence of a cross GCC and
binutils and should not be used for close-to-mainline kernel packages.

While at it, let's get rid of some unused dependencies and use
postmarketos-installkernel rather than Alpine's.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8124>

device/testing/linux-postmarketos-allwinner/APKBUILD

@@ -4,7 +4,7 @@ _flavor=postmarketos-allwinner
 _config="config-$_flavor.$CARCH"
 pkgname=linux-$_flavor
 pkgver=6.18.3_git20260105
-pkgrel=3
+pkgrel=4
 _tag="orange-pi-6.18-20260105-0049"
 pkgdesc="Kernel fork with Pine64 patches (megi's tree, slightly patched)"
 arch="aarch64 armv7"
@@ -13,20 +13,18 @@ license="GPL-2.0-only"
 makedepends="
 	bison
 	clang
-	devicepkg-dev
+	elfutils-dev
 	findutils
 	flex
-	gmp-dev
-	installkernel
+	linux-headers
 	lld
 	llvm
-	mpc1-dev
-	mpfr-dev
 	openssl-dev
+	pahole
 	perl
-	rsync
+	postmarketos-installkernel
+	python3
 	zstd
-	bash
 	"
 options="!strip !check !tracedeps
 	pmb:cross-native
@@ -58,35 +56,36 @@ esac
 
 prepare() {
 	default_prepare
-
-	REPLACE_GCCH=0 LLVM=1 \
-		. downstreamkernel_prepare
+	cp "$srcdir/config-$_flavor.$CARCH" .config
 }
 
 build() {
 	unset LDFLAGS
-	# V=1: workaround for pma#1990
 	make ARCH="$_carch" LLVM=1 \
 		KBUILD_BUILD_VERSION="$((pkgrel + 1 ))-$_flavor" \
 		CFLAGS_MODULE=-fno-pic \
-		DTC_FLAGS="-@" \
-		V=1
+		DTC_FLAGS="-@"
 }
 
 package() {
-	downstreamkernel_package "$builddir" "$pkgdir" "$_carch" "$_flavor"
-
-	make -j1 modules_install dtbs_install \
+	make dtbs_install modules_install zinstall \
 		ARCH="$_carch" \
 		LLVM=1 \
+		INSTALL_PATH="$pkgdir/boot" \
 		INSTALL_MOD_STRIP=1 \
 		INSTALL_MOD_PATH="$pkgdir"/usr \
 		INSTALL_DTBS_PATH="$pkgdir/boot/dtbs"
+
+	rm -f "$pkgdir"/usr/lib/modules/*/build "$pkgdir"/usr/lib/modules/*/source
+
+	install -D "$builddir"/include/config/kernel.release \
+		"$pkgdir"/usr/share/kernel/"$_flavor"/kernel.release
 }
+
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