gPXE is an open source network bootloader which I have been using to boot of iSCSI targets.
In order to get gPXE onto a bootable USB drive you must first create a custom image using their ROM-o-matic. Here you can choose the appropriate NIC and add additional options or an embedded script that will run automatically. You will want to choose “USB Keychain disk image (.usb)’ as the output format.
Rename the downloaded gpxe-xxxx.usb to gpxe-xxxx.img
Then download and run Win32DiskImager and point the application to gpxe-xxxx.img
Chose the device that corresponds to your USB flash drive and press ‘Write’.
Now all you have to do is plug the USB drive into the destination and boot from the USB device.