CMakeLists.txt: drop the hard-coded entry address in favor of a lookup of the .app_bin section in the map file

CMakeLists.txt

 	# Build the bootloader with the sections to fill in
 	pico_set_linker_script(picowota ${PICOWOTA_SRC_DIR}/bootloader_shell.ld)
 
-	# TODO: The hard-coded address here is a bit nasty
 	add_custom_target(${NAME}_hdr DEPENDS ${APP_BIN})
-	add_custom_command(TARGET ${NAME}_hdr DEPENDS ${APP_BIN}
-		COMMAND ${PICOWOTA_SRC_DIR}/gen_imghdr.py -a 0x1005B000 ${APP_BIN} ${APP_HDR_BIN}
+	add_custom_command(TARGET ${NAME}_hdr DEPENDS ${APP_BIN} picowota
+		COMMAND ${PICOWOTA_SRC_DIR}/gen_imghdr.py --map ${PICOWOTA_BIN_DIR}/picowota.elf.map --section .app_bin ${APP_BIN} ${APP_HDR_BIN}
 	)
 
 	add_custom_target(${COMBINED} ALL)