kernel-headers: fix modpost build in postinst, add arch tools

- Use 'make prepare' with 120s timeout to build modpost and all
  required files for external module compilation. Timeout is needed
  because 'make prepare' has a recursive call that creates infinite
  loop in headers-only tree, but modpost is built early before the
  recursion starts.
- Add verification that modpost was built successfully
- Include arch/${SRC_ARCH}/tools/ in headers package (contains .awk
  scripts like gen-cpucaps.awk needed by 'make prepare' on arm64)
- Filter out excessive "overriding recipe" warnings from output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: iav

lib/functions/compilation/kernel-debs.sh

@@ -409,7 +409,8 @@ function kernel_package_callback_linux_headers() {
 		[[ -d "arch/${SRC_ARCH}" ]] && {
 			find "arch/${SRC_ARCH}" -name module.lds -o -name Kbuild.platforms -o -name Platform
 			# shellcheck disable=SC2046 # I need to expand. Thanks.
-			find $(find "arch/${SRC_ARCH}" -name include -o -name scripts -type d) -type f
+			# include 'tools' for arch-specific scripts like gen-cpucaps.awk needed by 'make prepare'
+			find $(find "arch/${SRC_ARCH}" -name include -o -name scripts -o -name tools -type d) -type f
 			find arch/${SRC_ARCH}/include -type f
 		}
 		find Module.symvers include scripts -type f
@@ -534,16 +535,20 @@ function kernel_package_callback_linux_headers() {
 			echo "Configuring kernel-headers (${kernel_version_family}) - please wait ..."
 			make ARCH="${SRC_ARCH}" olddefconfig
 
-			echo "Compiling kernel-headers scripts (${kernel_version_family}) using \$NCPU CPUs - please wait ..."
-			make ARCH="${SRC_ARCH}" -j\$NCPU scripts
-
-			echo "Compiling kernel-headers scripts/mod (${kernel_version_family}) using \$NCPU CPUs - please wait ..."
-			make ARCH="${SRC_ARCH}" -j\$NCPU M=scripts/mod/
+			echo "Preparing kernel-headers for module builds (${kernel_version_family}) using \$NCPU CPUs - please wait ..."
+			# 'make prepare' builds scripts and scripts/mod (modpost). Use timeout because
+			# prepare has a recursive call that creates infinite loop in headers-only tree.
+			# modpost is built early, before the recursion starts.
+			timeout 120 make ARCH="${SRC_ARCH}" -j\$NCPU prepare 2>&1 | grep -v "^scripts/Makefile.build:.*warning: overriding recipe" || true
+			# Verify modpost was built successfully
+			if [ ! -x scripts/mod/modpost ]; then
+				echo "ERROR: modpost was not built!" >&2
+				exit 1
+			fi
 
 			echo "Compiling resolve_btfids tools for assigning stable BTF type IDs to kernel symbols"
 			make ARCH="${SRC_ARCH}" -j\$NCPU tools/bpf/resolve_btfids
 
-			# make ARCH="${SRC_ARCH}" -j\$NCPU modules_prepare # depends on too much other stuff.
 			echo "Done compiling kernel-headers (${kernel_version_family})."
 		EOT_POSTINST