swan_r5 improvements

[m-mcgowan]

We've added a few improvements to the Swan R5 (STM32L4R5) board that we hope can be included in the next release. This PR brings these changes:

• increases filesystem to 1MB by using bank2 of flash on the L4 as storage for that
• add an ext object on the board that contains the expanded set of pins on Swan from the castellated edges.
• Builds with the UF2 bootloader offset by default.
• adds the UF2 family ID for the STM32L4 and supported STM32 devices
• based on main. (not 7.3.x. despite the branch name)

Bootloader Installation

A new change in this PR is that the build default configuration makes provisions for a UF2 bootloader.

Here's how to install the bootloader

1. download the bootloader file tinyuf2-swan_r5.bin

2. Then put your Swan R5 into boot mode by holding the BOOT button, while pressing and releasing the RESET button.

3. Finally, run the command

dfu-util -s 0x8000000:leave -a 0 -D tinyuf2-swan_r5.bin

The Circuit Python binary can then be flashed using make flash or by dropping the UF2 image into the SWANBOOT drive.

[tannewt]

Thank you!