openssl: fix uci config for built-in engines

openssl: fix uci config for built-in engines

Built-in engine configs are added in libopenssl-conf/install stage
already, postinst/add_engine_config is just duplicating them, and
due to the lack of `config` header it results a broken uci config:

> uci: Parse error (invalid command) at line 3, byte 0

```
config engine 'devcrypto'
        option enabled '1'
engine 'devcrypto'
        option enabled '1'
        option builtin '1'
```

Add `builtin` option in libopenssl-conf/install stage and remove
duplicate engine configuration in postinst/add_engine_config to
fix this issue.

Fixes: 0b70d55a64c39d ("openssl: make UCI config aware of built-in engines")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>