refactor(boards): Adjust XIAO RP2040 override names, bootloader support

Adjust the .conf/.overlay files to match the proper naming for the
XIAO rp2040 board. Also add the necessary Kconfig/DTS bits for
supporting bootloader using retained memory/boot mode retention.

app/boards/xiao_rp2040.conf

@@ -9,3 +9,7 @@ CONFIG_SETTINGS_NVS=y
 CONFIG_FLASH=y
 CONFIG_FLASH_PAGE_LAYOUT=y
 CONFIG_FLASH_MAP=y
+
+CONFIG_RETAINED_MEM=y
+CONFIG_RETENTION=y
+CONFIG_RETENTION_BOOT_MODE=y

app/boards/xiao_rp2040.overlay

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: MIT
  */
 
+#include <arm/raspberrypi/rp2040-boot-mode-retention.dtsi>
+
 &xiao_serial { status = "disabled"; };
 
 &code_partition {