the luci interface somehow does a bad job at this so i decided to do this manually..
if you want to give it a try in the UI go to your interfaces, click edit, scroll down and find “dhcp”, switch to advanced tab and enter “serveraddress,10.0.0.100” and “filename,undionly.kpxe” to DHCP-Options (where the ip and the file should be adjusted to your needs.
ssh to your openwrt box!
add the following lines
config boot 'linux'
option filename 'ipxe.kpxe'
option serveraddress '10.0.0.183'
option servername 'life'
save and restart dhcp