FMUv3: Use its own proper prototype for the image

cmake/configs/nuttx_px4fmu-v3_default.cmake

@@ -2,6 +2,7 @@
 # FMUv3 is FMUv2 with access to the full 2MB flash
 set(BOARD px4fmu-v2 CACHE string "" FORCE)
 set(FW_NAME nuttx_px4fmu-v3_default.elf CACHE string "" FORCE)
+set(FW_PROTOTYPE px4fmu-v3 CACHE string "" FORCE)
 set(LD_SCRIPT ld_full.script CACHE string "" FORCE)
 
 include(nuttx/px4_impl_nuttx)