Implement IPv4 stack and ip app (AI)

- Created src/ipv4.h: IPv4 header struct, protocol numbers, checksum,
  address conversion (ipv4_aton/ntoa), send/receive/routing API,
  protocol handler registration
- Created src/ipv4.c: packet construction with header checksum,
  simple routing (direct subnet + gateway), incoming packet validation
  and dispatch to registered protocol handlers
- Created apps/ip/ip.c: displays network interface config from
  /sys/net (MAC, link, IP, netmask, gateway); supports 'ip set'
  to configure interface via sysfs writes
- Added ipv4.c to kernel build, kernel calls ipv4_init() at boot
- Tested: clean boot, IPv4 initialized, ip app in CPIO

README.md

@@ -71,7 +71,7 @@ Once a task is completed, it should be checked off.
 - [x] Create a network driver for the NE2000 NIC.
 - [x] Create a network driver for the 3C509B NIC. It should only support RJ45 and 10base-T.
 - [x] Create an ethernet subsytsem. Each ethernet device should be shown as a character device with the name `ethN`.
-- [ ] Create a IPv4 stack. Create the `ip` app that shows curernt IPv4 configuration. It should read this information from `/sys`
+- [x] Create a IPv4 stack. Create the `ip` app that shows curernt IPv4 configuration. It should read this information from `/sys`
 - [ ] Create a ARP subsystem. Create the `arp` command that shows current ARP tables. Again, this info should be found in `/sys`
 - [ ] Create a DHCP subsystem. Create the `dhcp` command to show current DHCP status information.
 - [ ] Create a UDP and TCP stack.