for what you need, it sounds like you should try out archlinux, there is a distro on the main page for the download for it.
Designing the kernel and filesystem is a bit difficult, much more than most people would even want to try. If you feel brave enough, I have a guide on how to design a kernel for a netbook using the raspberry pi softfloat version right here:
http://cae2100.wordpress.com/2013/05/31 ... a-netbook/
Just change "make uImage ARCH=arm" to "make ARCH=arm" and it probably will work fine to boot the raspberry pi up.
If it isnt smoking and blown into pieces, it's not pushed far enough yet.