[nrf noup] boards/thingy53_nrf5340_cpuapp: defaulat to LTO

make linking time optimization default for the target.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
(cherry picked from commit 473f7d7)

Author: nvlsianpu

boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf

@@ -73,3 +73,6 @@ CONFIG_FLASH_SIMULATOR_STATS=n
 # Enable custom command to erase settings partition.
 CONFIG_ENABLE_MGMT_PERUSER=y
 CONFIG_BOOT_MGMT_CUSTOM_STORAGE_ERASE=y
+
+CONFIG_ISR_TABLES_LOCAL_DECLARATION=y
+CONFIG_LTO=y