build-hafnium: 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 to pick the correct
openssl  installation as per the execution environment.

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: Ia1e2548c311bb58e80d4eef641f329e92ef2f388