Ignore nuttx files for Mbed-OS builds

Signed-off-by: George Beckstein <george.beckstein@gmail.com>
diff --git a/.mbedignore b/.mbedignore
index ca5d31c..06a173e 100644
--- a/.mbedignore
+++ b/.mbedignore
@@ -3,6 +3,7 @@
 boot/zephyr/*
 boot/cypress/*
 boot/espressif/*
+boot/nuttx/*
 ci/*
 docs/*
 ptest/*