Add espressif directory to .mbedignore

The old mbed CLI 1 tool, currently used to compile the MCUboot port for
Mbed™ OS, compiles all files unless otherwise specified in .mbedignore.
The espressif directory in the boot folder was added to the ignore file
to prevent failed builds.

Signed-off-by: Sudarshan Sreeram <sudarshan.sreeram@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/.mbedignore b/.mbedignore
index bd68385..ca5d31c 100644
--- a/.mbedignore
+++ b/.mbedignore
@@ -2,6 +2,7 @@
 boot/mynewt/*
 boot/zephyr/*
 boot/cypress/*
+boot/espressif/*
 ci/*
 docs/*
 ptest/*
@@ -16,4 +17,4 @@
 ext/nrf/*
 ext/tinycrypt/tests/*
 ext/tinycrypt/*
-ext/tinycrypt-sha512/*
\ No newline at end of file
+ext/tinycrypt-sha512/*