Add Buildroot instructions to the IR Yocto README

Update the instructions in the IR Yocto README to be more in sync with the
IR Guide[1] and thus detail how to build the firmware and Qemu with
Buildroot.

[1]: https://developer.arm.com/documentation/DUI1101/2-0/Run-the-ACS-IR-image-on-QEMU



Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>