run_qemu.sh: refactor loader.conf logic to disable and append

Instead of editing in place, comment out and append. This preserves
"history", especially when building in incremental mode.

This is the "zero functional change" part of the QEMU / KVM / breakpoint
PR #135 for issue #11

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

run_qemu.sh

@@ -937,13 +937,25 @@ update_rootfs_boot_kernel()
 	sudo cp "$builddir/mkosi.extra/boot/vmlinuz-$kver" "$builddir/mnt/run-qemu-kernel/$kver/vmlinuz"
 
 	defconf="$builddir/mnt/loader/loader.conf"
+	local loader_timeout=4
+
+	# mkosi->"bootctl install ..." creates a stub loader.conf.
+	# It can also come from a previous run of this script.
 	if [ -f "$defconf" ]; then
-		sudo sed -i -e 's/^#.*timeout.*/timeout 4/' "$defconf"
-		sudo sed -i -e '/default.*/d' "$defconf"
-	else
-		echo "timeout 4" | sudo tee "$defconf"
+		# Comment out existing values. Last value seems to win but it's not
+		# documented in "man loader.conf" so let's not rely on it
+		sudo sed -i -e 's/^[[:blank:]]*timeout\(.*\)/# timeout \1/' "$defconf"
+		sudo sed -i -e 's/^[[:blank:]]*default\(.*\)/# default \1/' "$defconf"
 	fi
-	echo "default run-qemu-kernel-$kver.conf" | sudo tee -a "$defconf"
+
+	{
+	    generatedfrom_header "update_rootfs_boot_kernel() >> $defconf"
+            cat <<EOF
+timeout $loader_timeout
+default run-qemu-kernel-$kver.conf
+
+EOF
+	}  | sudo tee -a "$defconf"
 
 	[[ "$_arg_legacy_bios" == 'on' ]] || install_opt_efi_shell