I am not entirely sure on the syntax, but you'd need to configure 209 to specify the configuration file and 210 to specify the directory location
configure vlan dhcp-option code 209 string "boot.cfg"
configure vlan dhcp-option code 210 string ".../isolinux/"Again, I'd need to confirm the last two lines
I actually didn't read you last question correctly, so the above response was to "how do you configure the bootfile". Let me not speculate, I'll try and see