Edit the file /etc/init.d/dnsmasq to add the right location and arguments:
start() { include /lib/network scan_interfaces config_load /var/state/network config_load dhcp # args="" args=" --enable-tftp --tftp-root=/mnt/usbdrive --dhcp-boot=/mnt/usbdrive/pxelinux.0"
Then type the following commands on the command line:
root@OpenWrt:~# uci set dhcp.dnsmasq.enable_tftp=1 root@OpenWrt:~# uci set dhcp.dnsmasq.tftp_root=/mnt/usbdrive root@OpenWrt:~# uci set dhcp.dnsmasq.dhcp_boot=/mnt/usbdrive/pxelinux.0 root@OpenWrt:~# uci commit dhcp root@OpenWrt:~# /etc/init.d/dnsmasq restart