Not any more since ca. Linux 3.14 (where they introduced DT for ARM). You just need a different "device tree" file which is more or less a compiled hardware description. The uImage/zImage and the kernel modules are quite universal nowadays and can be swapped around. The Pyra currently uses the "Letux" kernel which also runs on the OpenPandora. And they are different ARM (OMAP3 vs. OMAP5). So you just need to make U-Boot load either pyra.dtb or pandora.dtb and the otherwise identical system image boots on both. Unless you use special features of one SoC the other one doesn't have.