uhttpd: support using OpenSSL for certificate generation

uhttpd: support using OpenSSL for certificate generation

Support the usage of the OpenSSL command-line tool for generating
the SSL certificate for uhttpd. Traditionally 'px5g' based on
PolarSSL (or mbedTLS in LEDE), has been used for the creation.

uhttpd init script is enhanced by adding detection of an installed
openssl command-line binary (provided by 'openssl-util' package),
and if found, the tool is used for certificate generation.

Note: After this patch the script prefers to use the OpenSSL tool
if both it and px5g are installed.

This enables creating a truly OpenSSL-only version of LuCI
without dependency to PolarSSL/mbedTLS based px5g.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>