zephyr: Fall back to minimal C library

Changes back to the minimal C library instead of picolibc to
reduce flash usage

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index a6da049..851c133 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -33,3 +33,5 @@
 CONFIG_MCUBOOT_LOG_LEVEL_INF=y
 ### Decrease footprint by ~4 KB in comparison to CBPRINTF_COMPLETE=y
 CONFIG_CBPRINTF_NANO=y
+### Use the minimal C library to reduce flash usage
+CONFIG_MINIMAL_LIBC=y