Why Lower Layer(ll_usart & ll_gpio)is used in the Open bootloader project? can we use hal_uart & hal_gpio for this project?

[Shilpashree Madhu]

I'm doing an Open bootloader project for STM32L4R5ZI MCU, I don't want to use lower layer configurations for Usart, so how should I adapt the open bootloader project for my controller, what are the changes i need to do?