fix dependencies (again)

The dependencies should be between targets, not between commands and files.
Target dependencies on commands are not what they seem to be, and don't work.

Also see question, answer and comments on https://stackoverflow.com/questions/4010212/cmake-struggling-with-add-custom-command-dependencies

CMakeLists.txt

@@ -141,14 +141,14 @@ function(picowota_build_combined NAME)
 	# Build the bootloader with the sections to fill in
 	pico_set_linker_script(picowota ${PICOWOTA_SRC_DIR}/bootloader_shell.ld)
 
-	add_custom_target(${NAME}_hdr DEPENDS ${NAME})
-	add_custom_command(TARGET ${NAME}_hdr DEPENDS ${NAME} picowota
+	add_custom_target(${NAME}_hdr DEPENDS ${NAME} picowota)
+	add_custom_command(TARGET ${NAME}_hdr
 		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(${COMBINED} ALL)
 	add_dependencies(${COMBINED} picowota ${NAME}_hdr)
-	add_custom_command(TARGET ${COMBINED} DEPENDS ${NAME}_hdr
+	add_custom_command(TARGET ${COMBINED}
 		COMMAND ${CMAKE_OBJCOPY}
 			--update-section .app_hdr=${APP_HDR_BIN}
 			--update-section .app_bin=${APP_BIN} ${PICOWOTA_BIN_DIR}/picowota.elf ${COMBINED}.elf