px5g-mbedtls: add subjectAltName and extendedKeyUsage to SSL certs

px5g-mbedtls: add subjectAltName and extendedKeyUsage to SSL certs

To better acommodate with the current browsers' requirements, also
self-signed certificates should have subjectAltName and
extendedKeyUsage defined in the self-signed x509 SSL certificates.

The following case sensitive options are now possible:
-addext subjectAltName=DNS:...
-addext subjectAltName=EMAIL:...
-addext subjectAltName=IP:...
-addext subjectAltName=URI:...
-addext extendedKeyUsage=serverAuth OR -addext extendedKeyUsage=any

Initial draft by Paul Donald <newtwen@gmail.com>

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Link: https://github.com/openwrt/openwrt/pull/15366
Signed-off-by: Robert Marko <robimarko@gmail.com>