Create a new make rule for checking out submodules.

[kapacuk]

Currently, if you do a fresh checkout of the tinyusb project and try to build a FreeRTOS example you get an error like this on the first run:

......
LINK _build/seeeduino_xiao/seeeduino_xiao-cdc_msc_freertos.elf
arm-none-eabi-gcc: error: _build/seeeduino_xiao/obj/lib/FreeRTOS-Kernel/list.o: No such file or directory
arm-none-eabi-gcc: error: _build/seeeduino_xiao/obj/lib/FreeRTOS-Kernel/queue.o: No such file or directory
arm-none-eabi-gcc: error: _build/seeeduino_xiao/obj/lib/FreeRTOS-Kernel/tasks.o: No such file or directory
arm-none-eabi-gcc: error: _build/seeeduino_xiao/obj/lib/FreeRTOS-Kernel/timers.o: No such file or directory

You have to run make again, which is a nuisance.

When you run make the first time the files list.c, queue.c, tasks.c and timers.c don't exist under lib/FreeRTOS-Kernel, so there are no rules to build the corresponding *.o files.

This patch creates a new rule for checking out submodules, instead of doing it in the $(shell) function. The rule initialises every submodule directory and create an empty submodule.mk file in it. These empty files are included into the main makefile, forcing make to re-read everything and rebuild all rules after this stage. As a result, make works successfully on the first run.

[kapacuk]

hm.. let me fix that build failure first.

[kapacuk]

This does not work very well with make -j. Back to the drawing board..

[hathach]

yeah, the fetch submodules on-the-fly do not work well with -j option. And gcc may try to split task and I don't know, it doesn't seem to work well with dramatics file changes, similar to make -j clean all won't work. I was thinking of having submodule target, though I think just press up for previous target is much quicker. This is one-time issue, not a deal breaker though. Don't spend too much time on it :)