Start on submodule manager, add STM32U0 submodule

Author: multiplemonomials

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "doxygen-awesome-css"]
 	path = tools/doxygen-awesome-css
 	url = https://github.com/jothepro/doxygen-awesome-css.git
+[submodule "targets/TARGET_STM/TARGET_STM32U0/STM32Cube_FW/stm32u0xx-hal-driver"]
+	path = targets/TARGET_STM/TARGET_STM32U0/STM32Cube_FW/stm32u0xx-hal-driver
+	url = https://github.com/STMicroelectronics/stm32u0xx-hal-driver.git

targets/TARGET_STM/TARGET_STM32U0/STM32Cube_FW/CMakeLists.txt

@@ -1,6 +1,8 @@
 # Copyright (c) 2025 Jan Kamidra
 # SPDX-License-Identifier: Apache-2.0
 
+mbed_setup_submodule(stm32u0xx-hal-driver CHECK_FILE Src/stm32u0xx_hal.c IF_LABEL MCU_STM32U0)
+
 add_library(mbed-stm32u0cube-fw INTERFACE)
 
 target_sources(mbed-stm32u0cube-fw