sudo apt-get update
sudo apt-get upgrade

$ sudo apt-get install syslinux -y
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package syslinux is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'syslinux' has no installation candidate

$ mkwinpeimg --windows-dir=/mnt/windows7 win7pe.img
ERROR: To make a bootable disk image of Windows PE, we need the "syslinux"
program, but it doesn't seem to be installed.  Please install the "syslinux"
package to continue, or try using the --iso option to make an ISO image instead
of a disk image.