Merge c4a2086244 into d403044f76

Add `build_unflags = -fno-lto` to remove ESP-IDF's default LTO-off flag,
and add `-flto` to build_flags.

| Metric        | Without LTO          | With LTO             | Delta        | % Change |
|---------------|----------------------|----------------------|--------------|----------|
| firmware.bin  | 6,371,440 B          | 6,241,072 B          | -130,368 B   | -2.0%    |
| firmware.elf  | 39,599,948 B         | 22,893,308 B         | -16,706,640 B| -42.2%   |
| Flash used    | 6,218,928 B (94.9%)  | 6,117,722 B (93.3%)  | -101,206 B   | -1.6%    |
| RAM used      | 106,508 B (32.5%)    | 105,964 B (32.3%)    | -544 B       | -0.5%    |

platformio.ini

@@ -18,7 +18,9 @@ board_upload.flash_size = 16MB
 board_upload.maximum_size = 16777216
 board_upload.offset_address = 0x10000
 
+build_unflags = -fno-lto
 build_flags =
+  -flto
   -DARDUINO_USB_MODE=1
   -DARDUINO_USB_CDC_ON_BOOT=1
   -DMINIZ_NO_ZLIB_COMPATIBLE_NAMES=1