sysconfig: fix paths for ubuntu versions before 20.04 (fix #3)

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

builder.go

@@ -56,13 +56,13 @@ ff02::3 ip6-allhosts
   SAY Now booting the kernel from SYSLINUX...
  LABEL linux
   KERNEL /boot/vmlinuz-virt
-  APPEND ro root=UUID=%s rootfstype=ext4 initrd=/boot/initramfs-virt console=ttyS0,115200 
+  APPEND ro root=UUID=%s rootfstype=ext4 initrd=/boot/initramfs-virt console=ttyS0,115200
 `
 	syslinuxCfgCentOS = `DEFAULT linux
   SAY Now booting the kernel from SYSLINUX...
  LABEL linux
   KERNEL /boot/vmlinuz
-  APPEND ro root=UUID=%s initrd=/boot/initrd.img net.ifnames=0 console=tty0 console=ttyS0,115200n8 
+  APPEND ro root=UUID=%s initrd=/boot/initrd.img net.ifnames=0 console=tty0 console=ttyS0,115200n8
 `
 )
 
@@ -78,6 +78,8 @@ var (
 		"/usr/share/syslinux/mbr.bin",
 		// centos path
 		"/usr/share/syslinux/mbr.bin",
+		// archlinux path
+		"/usr/lib/syslinux/bios/mbr.bin",
 	}
 )
 
@@ -322,7 +324,11 @@ func (b *builder) installKernel(ctx context.Context) error {
 	var sysconfig string
 	switch b.osRelease.ID {
 	case ReleaseUbuntu:
-		sysconfig = syslinuxCfgUbuntu
+		if b.osRelease.VersionID < "20.04" {
+			sysconfig = syslinuxCfgDebian
+		} else {
+			sysconfig = syslinuxCfgUbuntu
+		}
 	case ReleaseDebian:
 		sysconfig = syslinuxCfgDebian
 	case ReleaseAlpine: