Implement DHCP client subsystem and dhcp app (AI)

- Created src/dhcp.h: DHCP packet struct, lease info struct, message types,
  options codes, client states, discover/receive/get_lease API
- Created src/dhcp.c: DHCP client with DISCOVER/OFFER/REQUEST/ACK flow,
  manual IP+UDP header construction for broadcast, option parsing for
  subnet mask/router/DNS/lease time/server ID, lease table, auto-applies
  configuration to ethernet interface on ACK, sysfs /sys/dhcp/status
- Created apps/dhcp/dhcp.c: reads /sys/dhcp/status to display DHCP info
- Kernel calls dhcp_init() at boot
- Tested: clean boot, DHCP initialized, dhcp app in CPIO

README.md

@@ -73,7 +73,7 @@ Once a task is completed, it should be checked off.
 - [x] Create an ethernet subsytsem. Each ethernet device should be shown as a character device with the name `ethN`.
 - [x] Create a IPv4 stack. Create the `ip` app that shows curernt IPv4 configuration. It should read this information from `/sys`
 - [x] 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.
+- [x] Create a DHCP subsystem. Create the `dhcp` command to show current DHCP status information.
 - [ ] Create a UDP and TCP stack.
 - [ ] Implement a simple version of `ftp` and `wget`.
 - [ ] Create a graphics subsystem. It should provide functionality to switch between the normal text mode, and a graphics mode.