Adjust ltsp-build-client/Debian-custom/001-ltsp-setting. (Closes: #928756)

Use PXE option 'ipappend 2' for LTSP client boot. This option makes sure
that all DHCP server information is getting through to LTSP clients.
(LTSP used this option before, but switched to 'ipappend 3' during the
Buster development cycle to ease setups with ProxyDHCP.)