feat(buzzy): add buzzy board support with SPI flash, fix build warnings

- Register BOARD_ROOT in CMakeLists.txt and sysbuild/CMakeLists.txt
- Add boardRoots to VS Code settings for board picker
- buzzy.dts: add nordic,pm-ext-flash chosen, external-flash and i2s-audio aliases
- nrf52840dk overlay: add external-flash alias (mirrors qspi-flash)
- fs_mgmt/Kconfig: select SPI_NOR for BOARD_BUZZY, NORDIC_QSPI_NOR for DK
- fs_mgmt.c: use external-flash alias instead of qspi-flash
- audio.c: bound snprintf to avoid truncation warning
- prj.conf: remove STACK_SENTINEL (conflicts with MPU_STACK_GUARD)
- mcuboot.conf: remove UART_CONSOLE (no SERIAL on buzzy)
- Delete mcuboot.overlay (no serial recovery needed)

Both buzzy/nrf52840 and nrf52840dk/nrf52840 build cleanly with zero warnings.

firmware/libs/audio/src/audio.c

@@ -186,7 +186,8 @@ static int audio_select_random_to_buf(char *buf, size_t buf_size)
         {
             if (current_index == random_index)
             {
-                snprintf(buf, buf_size, "%s/%s", FS_AUDIO_PATH, entry.name);
+                snprintf(buf, buf_size, "%s/%.*s", FS_AUDIO_PATH,
+                         (int)(buf_size - sizeof(FS_AUDIO_PATH) - 1U), entry.name);
                 LOG_DBG("Selected random audio file: %s", buf);
                 found = true;
                 break;