build-tf-a: Improve SSL_DIR script varible assignment

SSL_DIR was always set to use host based setup. This causes container
based builds to use wrong openssl.
This patch adds improvements to setup proper PATH for correct openssl
installation.

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: I3c2ee0da555fecb791a87999d6f8670479ebf13f