linux-postmarketos-qcom-sm8550: dtbo with a no-op overlay, not empty

A zero-entry DT table (32 bytes) fails Motorola's fastboot pre-flash
validation. Build a real no-op overlay with the kernel's dtc, wrap it in
a 1-entry DT table padded to 4 KB. Still applies nothing to the base DTB.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QF5HebjUsRaxbW4cBX6HJP

device/testing/linux-postmarketos-qcom-sm8550/APKBUILD

@@ -5,7 +5,7 @@ maintainer="snlavaise <snlavaise@gmail.com>"
 _flavor="postmarketos-qcom-sm8550"
 pkgname=linux-$_flavor
 pkgver=7.2
-pkgrel=3
+pkgrel=4
 pkgdesc="Mainline kernel for Qualcomm SM8550 (Snapdragon 8 Gen 2) + Motorola rtwo"
 arch="aarch64"
 _carch="arm64"
@@ -80,14 +80,6 @@ package() {
 	install -Dm644 "$builddir/arch/$_carch/boot/Image" \
 		"$pkgdir/boot/vmlinuz"
 
-	# Empty Android DT-overlay table (magic d7b7ab1e, 0 entries). The
-	# bootloader would otherwise apply the stock kalama dtbo overlay onto
-	# our mainline base DTB - the phandles don't match, the merged DTB is
-	# corrupt and the kernel panics. Flash this to the dtbo partition.
-	mkdir -p "$pkgdir"/boot
-	printf '\xd7\xb7\xab\x1e\x00\x00\x00\x20\x00\x00\x00\x20\x00\x00\x00\x20\x00\x00\x00\x00\x00\x00\x00\x20\x00\x00\x08\x00\x00\x00\x00\x00' \
-		> "$pkgdir"/boot/dtbo.img
-
 	make modules_install dtbs_install \
 		ARCH="$_carch" \
 		LLVM=1 \
@@ -95,6 +87,27 @@ package() {
 		INSTALL_MOD_STRIP=1 \
 		INSTALL_DTBS_PATH="$pkgdir"/boot/dtbs
 
+	# Android DT-overlay image with a single no-op overlay. The bootloader
+	# would otherwise apply the stock kalama dtbo overlay onto our mainline
+	# base DTB - the phandles don't match, the merged tree is corrupt and
+	# the kernel panics. A no-op overlay keeps the base DTB intact. (A
+	# zero-entry table fails Motorola's pre-flash validation.)
+	mkdir -p "$pkgdir"/boot
+	printf '/dts-v1/;\n/plugin/;\n/ { };\n' > "$srcdir"/empty-overlay.dts
+	scripts/dtc/dtc -@ -O dtb -o "$srcdir"/empty.dtbo "$srcdir"/empty-overlay.dts
+	python3 - "$srcdir/empty.dtbo" "$pkgdir/boot/dtbo.img" <<-'EOF'
+		import struct, sys
+		fdt = open(sys.argv[1], "rb").read()
+		hdr_sz = ent_sz = 32
+		ent_off = hdr_sz + ent_sz
+		hdr = struct.pack(">IIIIIIII", 0xd7b7ab1e, ent_off + len(fdt),
+				 hdr_sz, ent_sz, 1, hdr_sz, 2048, 0)
+		ent = struct.pack(">IIIIIIII", len(fdt), ent_off, 0, 0, 0, 0, 0, 0)
+		blob = hdr + ent + fdt
+		blob += b"\x00" * ((4096 - len(blob) % 4096) % 4096)
+		open(sys.argv[2], "wb").write(blob)
+	EOF
+
 	rm -f "$pkgdir"/usr/lib/modules/*/build "$pkgdir"/usr/lib/modules/*/source
 
 	install -Dm644 "$builddir"/include/config/kernel.release \