Image builder - where is my rootfs?

I was trying to use the image builder for mips malta to build a cut-down openwrt image for development/testing to run via qemu.

(I had to fix issues with the missing feeds-default.conf first)

I run the build via:

make image PACKAGES="gdbserver strace"

The kernel image is created, and is bootable (I get a no rootfs error) but I can't find the rootfs anywhere in the build tree? The build seems to succeed since no errors are given, even with verbose mode (V=s).

What am I missing? Do I really need to drop back to building everything from source (if so, what is the point in the image builder)?