Add env stm32g0b1re btt xfer

[elimisback]

Add a new env for BTT SKR Mini E3 V3. This new env allows uploading firmware in platformio over usb.

Description
I got tired of having to pull out the sd card on my SKR MINI E3 V3 to update the firmware; therefore, I implemented the same feature as the creality_xfer for the BTT SKR MINI E3 V3. This involves adding a new env STM32G0B1RE_btt_xfer.

Requirements
Only useful for the BTT SKR MINI E3 V3 as it is the only board using STM32G0B1RE at this time. Additionally for the feature to work, BINARY_FILE_TRANSFER and CUSTOM_FIRMWARE_UPLOAD must already be enabled. If these features are not enabled, the user would be required to first upload using SD Card.

Benefits
Less removing and replacing of SD Card in hopes to preserve life of the slot

Configurations
It complies and works on my printer. Let me know if you need the configs, but I think this is pretty straight forward.

Related Issues
No

[thisiskeithb]

Additionally for the feature to work, BINARY_FILE_TRANSFER and CUSTOM_FIRMWARE_UPLOAD

You can set those as build flags within the environment or that will need to be documented somewhere since users aren’t going to go looking for this PR for instructions.

Having said that, I’d prefer a *_USB environment, similar to other SKR Mini boards, setup instead since that is more universal and in line with the rest of this series of boards.

[elimisback]

So looks like buildroot/share/scripts/upload.py line 203 -205 should tell the user to enable those features.

Additionally, I asked about the _USB environment and was told this chip doesn't support that feature. I unfortunately am not savy enough to know whether that is true or not.

[thinkyhead]

I've added a sanity-check in the usual place to deal with missing requirements.