[OE-core] Add LibreSSL support

[Ruslan Babayev]

Sorry I meant this:

PREFERRED_PROVIDER_openssl ?= "libressl"
PREFERRED_PROVIDER_openssl-native ?= "libressl-native"
PREFERRED_PROVIDER_nativesdk-openssl ?= "nativesdk-libressl"

Ruslan Babayev writes:

> Please find attached the patch to add support for LibreSSL.
>
> OpenSSL is still preferred, so you'd have add these to your local.conf
>
> PREFERRED_PROVIDER_openssl ?= "openssl"
> PREFERRED_PROVIDER_openssl-native ?= "openssl-native"
> PREFERRED_PROVIDER_nativesdk-openssl ?= "nativesdk-openssl"