Added HAL and Makefile for STM32L431

[Lakshay983]

PeripheralSOM is going to use the STM32L4 mcu, so we need to have that HAL code.

I made it an option in the makefile to select compiling something for STM32L4 by setting chip=L4 while making. I get a few warnings due to BSP.c but that's due to it only being made for the F4 I assume.

[Lakshay983]

Looks mostly good but the makefile for L4 needs to be modifed. See what was changed in the F4 makefile (takes in additional inputs, has sources and includes defined differently, compiled differently for a test and for a regular make (using main.c), etc.)

Ah I didn't see the F4 makefile was modified. I can go look through that.

[Lakshay983]

I need to fix the following before merging:

• [X] Heartbeat needs to work on hardware
• [X] Add L4 OpenOCD script
• [X] Purge CMSIS folder which has a lot of the L4 chips we don't need
• [X] Combine the L4 and F4 makefile, they are the same and it should just be an option we pass into the makefile (maybe this is a different PR)
• [X] Also the makefile variable name, "HAL" sucks
• [X] Add documentation on the makefile and how to set different options for make
• [X] Finish make help
• [X] Make a new folder for HwTests (change the makefile accordingly
• [X] Remove some of the redundant hal files like stm32l432, 33, and more

There will prob be a need for a better testing framework but that is smth for a different pr.

[IshDeshpa]

Just took a look at #54; sorry if most of these comments don't mean much in the context of those changes :')