Well, the main problem is that the hardware you are using will not support the grugq's image. According to the specs here
The hardware only has 4mb flash memory, and to run Tor alone you will need at least half a meg of binaries. Thats the reason most commercial routers will not run Tor even with firmware changes. Arch linux is an unusual choice for host OS, but the raspberry should have resources to support it. If you are willing to use openwrt, there is a good tutorial on the Tor project site:
to hardware router
There is a tutorial for running it based on debian also but not one for arch.
Installing openwrt will probably get you up and running the fastest. You can just login by ssh and update the package manager
#opkg update
install Tor (might as well try the alpha versions)
#opkg install tor-alpha
The login and overwrite your /etc directory with premade config files:
vanilla tor router /etc directory