lua-eco: rework SSL library dependency & selection

lua-eco: rework SSL library dependency & selection

Currently, lua-eco will add dependencies to all SSL libraries that are
selected, even though it will only use one of them.  That means that the
package downloaded from the regular repository will install OpenSSL,
wolfSSL and mbedTLS, even though it will only use OpenSSL.

Fix that by adding a built option so that the default can be changed at
build-time.  To maintain the author's intention, a default symbol is
computed based on what libraries are being built into the image, or just
selected as a module.  Originally, the order or preference was OpenSSL,
wolfSSL, then mbedTLS.

One change was made to the original order: if OpenSSL and wolfSSL are
both selected as module, and mbedTLS is not built into the image,
wolfSSL will be preferred over OpenSSL.  This is being done to keep the
package consistent with OpenWRT's selection of wolfSSL as the default
SSL library.  If they are both included in the image, then OpenSSL will
be preferred.

The order of preference is:
1. If at least one library is included in the image, use the first of
   OpenSSL, wolfSSL, and mbedTLS that is included in the image.
2. If at least one library is selected, but none included in the image,
   prefer wolfSSL, then OpenSSL, then mbedTLS.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>