Fix SKR 1.4 UART3 Environment

[thisiskeithb]

Description

PR #21254 introduced some new warnings & build errors for the SKR 1.4, so this corrects the platform line so the new LPC1768_btt_skr_v1_4 PIO environment compiles correctly.

H/T to @rhapsodyv for the fix.

Requirements

SKR 1.4 with new LPC1768_btt_skr_v1_4 PIO environment.

Benefits

No more "Can not remove temporary directory /github/Marlin/.pio/build. Please remove it manually to avoid build issues" or "Error: Invalid '/github/Marlin/platformio.ini' (project configuration file): 'No section: 'LPC1768''" warnings/errors.

Configurations

Any BOARD_BTT_SKR_V1_4 config.

Related Issues

#21259

[thisiskeithb]

What about the SKR 1.4 Turbo? It's identical in every way to the SKR 1.4, but has an LPC1769 instead of LPC1768 MCU.

Should we add an LPC1769_btt_skr_v1_4_turbo PIO environment?

[p3p]

Should we add an LPC1768_btt_skr_v1_4_turbo PIO environment?

Adding the extra envs is not the correct workaround for removing the board flags feature, all it does is complicate matters, I think using the platformio.ini features as mentioned in this comment #21254 (comment) would be cleaner.

[thisiskeithb]

Adding the extra envs is not the correct workaround for removing the board flags feature

I was going to mimic what was already merged, but I'm happy to make the change any way you prefer.

[thisiskeithb]

I think using the platformio.ini features as mentioned in this comment #21254 (comment) would be cleaner.

Looking at that again, it is!

The biggest advantage of that method is that it doesn’t invalidate all current documentation on which environment to use.