luci-ssl-openssl: use OpenSSL for key generation, drop px5g

Replace a dependency for px5g with openssl-util. Both LEDE and
Openwrt have now uhttpd versions that can generate keys with
OpenSSL cmd tools if px5g is not found. This change enables an
"OpenSSL-only" version of LuCI without mbedTLS/PolarSSL.

(Note: if px5g is found, uhttpd will prefer that.)

Relevant commits in uhttpd:

LEDE:
https://github.com/lede-project/source/commit/3c4858eeb2bbb3107f87bb3be07d5c172c8e0ef9
https://github.com/lede-project/source/commit/82132540a3efbc98f8f4379b26d4b4541013e69d
https://github.com/lede-project/source/commit/9097dc5ad844c336020be11085e1c8c80390ac9c

Openwrt:
https://github.com/openwrt/openwrt/commit/6ec32832d25335355d205c4601d4a8ca5dfd6808

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