[nrf fromlist] boot: zephyr: Cleanup RAM by default

Enable the CONFIG_MCUBOOT_CLEANUP_RAM by deafult to improve security
of the default configuration.

Upstream PR: mcu-tools/mcuboot#2451

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

Author: MarekPieta

boot/zephyr/Kconfig

@@ -469,6 +469,7 @@ config MCUBOOT_CLEANUP_ARM_CORE
 config MCUBOOT_CLEANUP_RAM
 	bool "Perform RAM cleanup"
 	depends on CPU_CORTEX_M4 || CPU_CORTEX_M33
+	default y
 	help
 	  Sets contents of memory to 0 before jumping to application.