zephyr: disable multithreading

The bootloader is just using Zephyr as a HAL and doesn't require
thread scheduling. Build Zephyr in single-threaded mode and gain
some footprint savings.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index f3cffde..f6d9342 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -14,3 +14,5 @@
 
 ### Disable Bluetooth by default
 # CONFIG_BLUETOOTH is not set
+
+CONFIG_MULTITHREADING=n