[nrf fromlist] zephyr: Enable custom settings erase cmd on Thingy:53

Upstream PR:
mcu-tools/mcuboot#1136

Change introduces custom settings erase command on Thingy:53.

Jira: TG53-132

(cherry picked from commit deccdbb53f4b62fb75a83ecd408a3fdd6125cdee)
Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

Author: MarekPieta

boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf

@@ -75,3 +75,7 @@ CONFIG_BOOT_UPGRADE_ONLY=y
 CONFIG_FLASH_SIMULATOR=y
 CONFIG_FLASH_SIMULATOR_DOUBLE_WRITES=y
 CONFIG_FLASH_SIMULATOR_STATS=n
+
+# Enable custom command to erase settings partition.
+CONFIG_ENABLE_MGMT_PERUSER=y
+CONFIG_BOOT_MGMT_CUSTOM_STORAGE_ERASE=y