openssl: add PERLEXTERNAL path to test its existence

When upstream change is better to fail or removing the PERL5LIB
if they are not need anymore.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>