[stm32] Add FreeRTOS + LwIP submodules

These are ST's forks of each library. The changes from upstream seem
pretty minimal. The main change is the integration of the CMSIS-RTOSv2
API.

stm32_mw_freertos: v10.2.1
stm32_mw_lwip: v2.1.2

Change-Id: I5f8373083a43d0f7cba7d22f1786d9922048bb5e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/40041
Reviewed-by: Yecheng Zhao <zyecheng@google.com>
Commit-Queue: Varun Sharma <vars@google.com>
diff --git a/.gitmodules b/.gitmodules
index 3028ac5..be13cc0 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -25,3 +25,9 @@
 [submodule "third_party/stm32cubef4/stm32f4xx_hal_driver"]
 	path = third_party/stm32cubef4/stm32f4xx_hal_driver
 	url = https://github.com/STMicroelectronics/stm32f4xx_hal_driver.git
+[submodule "third_party/stm32cubef4/lwip"]
+	path = third_party/stm32cubef4/lwip
+	url = https://github.com/STMicroelectronics/stm32_mw_lwip.git
+[submodule "third_party/stm32cubef4/freertos"]
+	path = third_party/stm32cubef4/freertos
+	url = https://github.com/STMicroelectronics/stm32_mw_freertos.git