To get some funcionality, we will need a shell, i like bash a lot, so i will give that a try.
Bash, again, can be found at GNU, and is currently at version 2.05b.

Also bash has a configuration script for the build process. Because my router is a pentium machine, i used:
./configure --build=i586-pc-linux-gnu

After that, a simple make command started the building process.

The building process didn't took too long, and produced a 3 MB binary, which i thought is very big. Stripping debug information is a good thing to do: stip -g bash, which shrinks the file to 731 KB, that's better !

After checking with ldd, you can see that bash requires ncurses libraries, so that's the next thing to do.