fix: USB CDC/MDC on Cygnet

[m-mcgowan]

Fixes USB on Cygnet.

Major changes, enabling USB functionality:

• The L433 doesn't support USB OTG, hence the USB ISR has a different name with the L433 compared to the L4R5.
• The enable/disable interrupts code was failing and disabling all interrupts after a certain period because it was using non-atomic operations. I rewrote the code to use atomic operations so that interrupts are re-enabled correctly. (Edit: I'm not convinced the new code is actually atomic, but could have resulted in a compiler optimization error.)

Minor changes:

• Normalized the defines that differ by case (STM32L4R5xx / STM32L4R5XX) to use only the lowercase version, and similarly for the L433, which were only causing confusion.
• Tidied up some defines that were using preprocessor symbols in a way that implied they were guaranteed to be defined, being either 0 or 1, but in reality are only defined when targeting certain boards.
• indented init_usb_vbus_sense() to make the nested conditionals clearer to read.

[bsatrom]

Thanks @m-mcgowan! I'll test these changes today and let you know!