When you run
export TEMPLATECONF=meta-openbmc-machines/meta-openpower/meta-ibm/meta-palmetto/conf, you are telling the build system that you want to build images for the BMC processor used on the Palmetto machine (Aspeed AST2400). That image won't boot on a RaspberryPi. What you'll need to do is create a new layer in meta-openbmc-machines for a RaspberryPi that includes the raspberrypi layer, specifies the correct kernel device tree, etc. Take a look at the meta-ast2500-evb and https://github.com/Nuvoton-Israel/meta-bsp-nuvoton/tree/master/meta-openbmc-machines/meta-evb/meta-evb-nuvoton/meta-evb-npcm750/conf for examples of what a machine to support an evaluation board (evb) looks like.