STM32L4xx for significantly better USB support

[StefanBruensLC]

The USB OTG FS controller in the F401/F411 is severely crippled, as it support only 3 USB Endpoints per direction, which makes it impossible to implement more than 1 USB CDC interface (1 Bulk IN and 1 Interrupt IN required per interface).

The L4xx support 6 endpoints per direction, so up to 3 USB CDC could be supported, or a variety of other compositions.

The L4xx are mostly pin compatible with the F401/F411 (and are fully compatible with the BlackPill schematic).

Given the BluePill+ (which does not have these restrictions) is completely unavailable, this would be very welcome.