Just to wrap this up for anyone else coming here,
qoriq-composite-firmware will generate a set of files firmware_ls1043ardb_uboot_{nor,sd,nand}boot.img. They include RCW, BL*, U-Boot, some microcode, Kernel, device tree and a ram filesystem. The kernel/filesystem is a fit file that is pulled into memory then booted.
It is based on the components from fsl-image-mfgtool
I put the the SD version on a SD card and it did boot Ok
Building, using the recipes from @Chavira above, you can either
source setup-env -m ls1043ardb
which builds a system based on initscripts (49M) or you can use
source distro-setup-env -m ls1043ardb
which builds a slightly larger image (57M) based on systemd.
I have not worked out whether there is anything in 'distro' I need yet, but I'm guessing there are a few other differences.
One word of warning when using distro-setup-env. I build using the Yocto host toolchain (sources/poky/scripts/install-buildtools). The distro-setup-env builds a patched version of the Yocto pseudo tool which is quite old but has changes needed to run the Ubuntu apt tools. The version of pseudo used is too old to compile with the Yocto toolchain. Since for
qoriq-composite-firmware we don't need apt tools, we need to revert to the default Yocto version of pseudo to get it to build.
The files to drop are meta-nxp-desktop/recipes-devtools/pseudo/pseudo_git.bb and meta-nxp-desktop/dynamic-layers/core/recipes-devtools/pseudo/pseudo_%.bbappend