I've put together a quick patch to provide defaults for compiling Buildroot with the Raspberry Pi 2 as a target.
The changes are based on the current default Raspberry Pi configuration files provided by Buildroot, however they bump the version of the firmware that is used, and configure all compilation to be done with the ARMv7 instruction set, rather than ARMv6. I've also enabled SMP and NEON CPU instructions.
With these changes there should be performance improvements over using a stock ARMv6 Raspberry Pi buildroot on the RPi2. I have tested the resulting image but haven't done any benchmarking yet.
Update: I've amended this patch based on feedback from the buildroot mailing list. The main change is enabling EABIhf in buildroot. That patch is also now split in two, one for the defconfig, and the other for an updated rpi-firmware package.
Another update: This work along with the work of others has made it into buildroot as a supported device, these patches are no longer required.