scripts: use pkg-config to locate libcrypto

Otherwise build fails if the headers are not in the default location. While at
it also ask pkg-config for the libs, with fallback to the existing value.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Cc: stable@vger.kernel.org # 5.6.x
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>