zephyr: Disable power management

Power management requires multithreading and other kernel features that
are disabled in MCUboot, so disable it to avoid interrupts being
confused by power management code wrappers.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index 7368e02..60fd5e8 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -2,6 +2,7 @@
 CONFIG_SYS_LOG=y
 CONFIG_DEBUG=y
 CONFIG_SYSTEM_CLOCK_DISABLE=y
+CONFIG_SYS_POWER_MANAGEMENT=n
 
 CONFIG_MAIN_STACK_SIZE=10240
 CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"