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/boards/nrf52840dk_nrf52840.overlay

@@ -3,6 +3,7 @@
         nordic,pm-ext-flash = &mx25r64;
     };
     aliases {
+        external-flash = &mx25r64;
         qspi-flash = &mx25r64;
         i2s-audio = &i2s0;
     };