Fixed "illegal instruction" in libgcrypt with AES/CBC

That was visible at least in wpa_supplicant on networks with EAP auth
and AES cipher used. wpa_supplicant would fail in the handshake with
"illegal instruction".
Added README to libgcrypt fix
Merge branch 'master' into image-devtools
Enabling libgcrypt AES fix by default for devtools image