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>

.mbedignore

@@ -2,6 +2,7 @@ boot/boot_serial/*
 boot/mynewt/*
 boot/zephyr/*
 boot/cypress/*
+boot/espressif/*
 ci/*
 docs/*
 ptest/*