net/dhcp: Replace parse_dhcp_vendor() with find_dhcp_option()

net/dhcp: Replace parse_dhcp_vendor() with find_dhcp_option()

For proper DHCP support we will need to parse DHCP options from a packet
more often and at various places.

Refactor the option parsing into a new function, which will scan a packet to
find *a particular* option field. Use that new function in places where we
were dealing with DHCP options before.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>