SOC for router like a d-link?
There are lots from manufacturers like Broadcom, Atheros, Realtek, Ralink. Most modern routers are running a SoC with some external flash and RAM, and most are already running Linux. The dd-wrt, OpenWRT and Tomato third-party firmwares are all Linux-based.

You're better off getting an off-the-shelf router that has the firmware source code available, and simply study/customize it yourself.

EDIT: The D-link DIR-825 described is supported by OpenWRT (rev. B hardware only), which makes its source code available. It is theoretically possible to compile in other functionality so long as the end result image fits in the available flash. The specifics of BOOTP are better suited for another SE site.