init: Support parsing of IPv4 CIDR (`UK_NETDEV_IPV4_CIDR`)

init: Support parsing of IPv4 CIDR (`UK_NETDEV_IPV4_CIDR`)

This commit introduces support for parsing a statically assigned IP address
from the the netdev's extended information (einfo) field:
`UK_NETDEV_IPV4_CIDR`. The CIDR notation is provided in preference by
`lib/uknetdev` and must be read before looking into `UK_NETDEV_IPV4_ADDR`
and `UK_NETDEV_IPV4_MASK` fields.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #42