Flashing Process R7800.

Through recovery mode:

In order to flash kong openwrt build from dd-wrt.
Press reset button and hold it, power on and wait until power led
flashes white. Now you can use a tftp client to push the 
<model>-factory.img to the router. The unit will reboot by itself
and you should see openwrt webif.

Through Webif:

In order to flash kong openwrt from oem firmware, use oem
update page and upload <model>-factory.img

In order to update openwrt use <model>-sysupgrade.bin and flash via openwrt
update page.

Sourcecode:

https://github.com/ricsc/openwrt