build: allow to build LEDE on latest MacOS X

Latest Xcode doesn't include openssl anymore. To compile
mkimage from u-boot source you need SSL headers on your host.
This patch provides libressl host package for any Darwin
compilation. Unfortunately openssl from MacPorts can not be
used, as the installed headers in /opt/local are breaking
GDB compilation. Tested with a RB532 image build and resulting
kernel booted on a device via TFTP.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [fixes, dependencies]