Append the following to /etc/config/dhcp:
config boot 'linux'
option filename 'pxelinux.0'
option serveraddress 'YOUR.SERVER.IP.HERE'
option servername 'HP Deployment'
list dhcp_option '186,10.0.255.251'
list dhcp_option '187,8017'
list dhcp_option '252,\\boot\BCD'
~