fix(rootfs/qemu-static): prefer native COMPAT over existing qemu-arm registration

When CONFIG_COMPAT runs armhf binaries natively on aarch64 (≈10× faster
than qemu-user emulation), but qemu-arm is registered in kernel binfmt_misc,
the previous "trust existing setup" check returned early and the build
went through qemu anyway. Empirically: helios4 build 41 min via qemu vs
19 min native COMPAT on droid (RK3399).

Reorder the probes: COMPAT first; if it works and qemu-arm is enabled,
actively disable the binfmt_misc entry (echo 0 > /proc/.../qemu-arm).
The descriptor itself is left intact — only the active flag is toggled.

Gated by ARMBIAN_PREFER_NATIVE_ARMHF=no for explicit opt-out. Behaviour
preserved on hosts without COMPAT.

Assisted-by: Claude:claude-opus-4.7

Author: iav

lib/functions/rootfs/qemu-static.sh

@@ -194,26 +194,39 @@ function prepare_host_binfmt_qemu_cross() {
 
 function prepare_host_binfmt_qemu_cross_arm64_host_armhf_target() {
 	declare armhf_probe="/usr/arm-linux-gnueabihf/lib/ld-linux-armhf.so.3"
+	declare prefer_native="${ARMBIAN_PREFER_NATIVE_ARMHF:-yes}"
+
+	# Probe CONFIG_COMPAT first via direct exec of ld-linux-armhf. When
+	# COMPAT works, native armhf execution is ≈10× faster than qemu-user
+	# emulation, so we prefer it even if qemu-arm is already registered:
+	# kernel binfmt_misc routes armhf exec through qemu first, masking
+	# COMPAT, so the qemu-arm entry must be actively disabled.
+	#
+	# `arch-test arm` is unreliable here (probes ARMv5 EABI; COMPAT runs
+	# armhf — EABI v5+; empirically broken on Ubuntu Noble / Ampere CAX).
+	# ld-linux-armhf comes from gcc-arm-linux-gnueabihf, an armbian host
+	# build dep when target_arch=armhf|all.
+	if [[ "${prefer_native}" == "yes" ]] && [[ -x "${armhf_probe}" ]] &&
+		"${armhf_probe}" --help > /dev/null 2>&1; then
+		if [[ -e /proc/sys/fs/binfmt_misc/qemu-arm ]] &&
+			[[ "$(head -n1 /proc/sys/fs/binfmt_misc/qemu-arm 2> /dev/null)" == "enabled" ]]; then
+			display_alert "Disabling qemu-arm to use native COMPAT for armhf" "≈10× faster than qemu emulation" "info"
+			echo 0 > /proc/sys/fs/binfmt_misc/qemu-arm 2> /dev/null || true
+		else
+			display_alert "Host kernel can run armhf natively (CONFIG_COMPAT)" "no qemu-arm setup needed" "debug"
+		fi
+		return 0
+	fi
 
-	# If qemu-arm is already enabled in the kernel, the native probe
-	# below would route through qemu and lie about COMPAT. Trust the
-	# existing setup — admin or packaged service intended it.
+	# Native COMPAT unavailable (or opt-out via ARMBIAN_PREFER_NATIVE_ARMHF=no).
+	# Trust an existing qemu-arm registration — admin or packaged service
+	# intended it.
 	if [[ -e /proc/sys/fs/binfmt_misc/qemu-arm ]] &&
 		[[ "$(head -n1 /proc/sys/fs/binfmt_misc/qemu-arm 2> /dev/null)" == "enabled" ]]; then
 		display_alert "qemu-arm already enabled in binfmt_misc" "trusting existing setup" "debug"
 		return 0
 	fi
 
-	# No active qemu-arm route. Probe CONFIG_COMPAT directly. `arch-test
-	# arm` is unreliable here (probes ARMv5 EABI; COMPAT runs armhf —
-	# EABI v5+; empirically broken on Ubuntu Noble / Ampere CAX). Direct
-	# exec of ld-linux-armhf (from gcc-arm-linux-gnueabihf, an armbian
-	# host build dep when target_arch=armhf|all) is authoritative.
-	if [[ -x "${armhf_probe}" ]] && "${armhf_probe}" --help > /dev/null 2>&1; then
-		display_alert "Host kernel can run armhf natively (CONFIG_COMPAT)" "no qemu-arm setup needed" "debug"
-		return 0
-	fi
-
 	# ld-linux-armhf may be absent on cross builds whose target isn't
 	# armhf (gcc-arm-linux-gnueabihf isn't pulled in then). Fall back to
 	# arch-test to avoid degraded host-capability detection on those