The OpenEmbedded Prerequisites (OpenEmbedded (core) - Toradex System/Computer on Modules) for “Ubuntu 16.04 and 18.04 (64-bit)” include “libcrypto++9v5:i386”; however this package no-longer exists in the Ubuntu 18.04 packages.
Researching this package shows that libcrypto++9v5 (from 16.04) appears to be derived from version 5.6.1 of libcrypto++. I was able to find libcrypto++6 in 18.04 which appears to be derived from version 5.6.4 of libcrypto++. It’s a little confusing that libcrypto++6 is slightly newer than libcrypto++9v5.
I tried installing libcrypto++6:i386 and everything appears to build correctly. Could you confirm this should be correct; and perhaps update the instructions on the openembedded prerequisites page if so.