firewall: wait for PHY IRQ before starting stack

[nwf]

Depends on CHERIoT-Platform/cheriot-rtos#530
Work towards lowRISC/sonata-system#410

With this (and its dependency) in place, the first DHCP discover packet successfully egresses:

Firewall: Initialising network interface
Firewall: Link not ready; wait for first IRQ
Firewall: Link ready? true
vDHCPProcessEndPoint: enter 0
DHCP-socket[fe-7a]: DHCP Socket Create
prvCreateDHCPSocket[fe-7a]: open, user count 1
prvInitialiseDHCP: start after 25 ticks
vDHCP_RATimerReload: 25
vDHCPProcessEndPoint: exit 1
vDHCPProcessEndPoint: enter 1
vDHCPProcess: discover
vDHCPProcessEndPoint: exit 2
Firewall: Permitting broadcast UDP packet
Firewall: Permitting outbound IPv4 packet

and is replied to:

Firewall: DNS server address set to 0x1071dac
Ethernet Adaptor: Sending frame to IP task
vDHCPProcessEndPoint: enter 2
vDHCPProcess: offer ac1d0761ip for MAC address fe-7a