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/sysbuild/mcuboot.conf

@@ -1,6 +1,5 @@
 CONFIG_LOG=y
 # CONFIG_MCUBOOT_SERIAL=y
-CONFIG_UART_CONSOLE=y
 # CONFIG_SINGLE_APPLICATION_SLOT=n
 # CONFIG_MCUBOOT_INDICATION_LED=y
 # CONFIG_BOOT_SERIAL_CDC_ACM=y

firmware/sysbuild/mcuboot.overlay

@@ -1,13 +0,0 @@
-/ {
-  aliases {
-		mcuboot-button0 = &button0;
-		mcuboot-led0 = &led0;
-  };
-};
-
-/* Step 2.1 - Configure CDC ACM */
-&zephyr_udc0 {
-	cdc_acm_uart0: cdc_acm_uart0 {
-		compatible = "zephyr,cdc-acm-uart";
-	};
-};