package/mongoose: fix activation of openssl/mbedtls

MG_ENABLE_SSL and MG_SSL_IF have been dropped since version 7.0 and
https://github.com/cesanta/mongoose/commit/f2fba1d2004c5ddf2fc0a7ca8dc75b5f78feed85



So use the new MG_ENABLE_OPENSSL and MG_ENABLE_MBEDTLS variables

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>