base: Fix building of OpenSSL. See #73
base: Fix building of OpenSSL. See #73

This commit fixes the following error reported when building the base
image.

sh: 1: [: =: unexpected operator
sh: 1: [: =: unexpected operator

It turns out that (1) DEFAULT_DOCKCROSS_IMAGE is not set when building the
base image and (2) latest openssl will not be available in linux-x64 and
linux-x86 images.

thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross