Rename BTT things consistently, with SKR 2 based on MCU

[thinkyhead]

Based on #27301 by @thisiskeithb — Rename all the BigTreeTech stuff with a "BTT" prefix for consistency. Then, "BIGTREE_SKR_2_*" environments are renamed with the MCU name in front to follow the standard applied to boards that have more than one MCU variant.

[thisiskeithb]

In reply to #27307 (comment), I'm cool with the shorter BTT naming scheme done here. The full legal name for many companies would be a bit much 😃

Then, SKR 2 environments are renamed with the MCU name in front to follow the standard applied to boards that have more than one MCU variant.

I guess we can cross that bridge when it happens, but I'd like to standardize on using this MCU_oemname_feature_addons environment naming scheme from the get-go for all boards since using multiple processors for the same board may not be planned initially/happens later. Then it'd be easier to add additional environment defines & it'd really clear to anyone compiling, especially those ABM users.

[thinkyhead]

We get into a weird conundrum, or maybe it just points out an inconsistency in our environment inheritance… but we now have an environment for some MCU extending an environment for another variant of that MCU, and it's not always clear if the variant has extra features, extra pins, etc., just from the name, so it looks a little weird. Whereas, if the root environment for SKR 2 variants is named env:BTT_SKR_2 it more clearly relates to a family of boards with presumably some common ideas about layout and pin mapping, or if there are no commonalities, all the more reason to use a generic (non-MCU-specific) name in some cases. If users are using the markings on the chip as their best reference to tell which board variant they have, that's unfortunate, so if naming things by MCU helps identify boards more easily, so be it.

[thisiskeithb]

it's not always clear if the variant has extra features, extra pins, etc., just from the name

The DRY principle is great, but I'm fine spelling things out if it's not clear. I actually did this recently while helping someone with a new board design based on one of BTT's.

If users are using the markings on the chip as their best reference to tell which board variant they have, that's unfortunate, so if naming things by MCU helps identify boards more easily, so be it.

It is indeed unfortunate. We ask users to verify the MCU type constantly because board revisions / models are not always updated to reflect major board changes like swapping in a different MCU.