build(esp32s2/s3): drop unconditional -DUSB_VID/USB_PID injection (149-156 redefinition warnings per build)

[zackees]

Summary

On esp32s2 and esp32s3 builds, fbuild unconditionally injects -DUSB_VID=... and -DUSB_PID=... (and likely USB_MANUFACTURER/USB_PRODUCT) into the compile-flag set. The Arduino framework variant headers also #define these symbols, producing 149-156 redefinition warnings per build.

warning: "USB_VID" redefined
note: this is the location of the previous definition  <command-line>

Locations:

• framework-arduinoespressif32/.../variants/esp32s3/pins_arduino.h:7
• framework-arduinoespressif32/.../variants/lolin_s2_mini/pins_arduino.h:7 (also other S2 variants)

Evidence

• esp32s3 audit run: 156 hits
• esp32s2 audit run: 149 hits
• rgbw (esp32s3 underneath): 39 hits

This is noise that buries real warnings and slows CI log grep / diagnostics workflows.

Proposed fix

Mirror PlatformIO behavior: only inject -DUSB_VID/USB_PID when the user has explicitly set custom values in build_flags or the board JSON, otherwise let the variant pins_arduino.h define them. This requires the ESP32 flag builder (crates/fbuild-build/src/esp32/.../flag_* likely) to detect default-vs-override.

Acceptance: a clean esp32s3 build emits zero "USB_VID" redefined warnings; a user sketch with build_flags = -DUSB_VID=0xDEAD still wins.

[zackees]

Tracked under fbuild meta-audit: #408