dhcpd: Align makefile implementation

[zs39]

Summary

Improve the CMakeLists.txt to achieve the same compilation results as Makefile.

Impact

CMake compilation affecting dhcpd

Testing

CMake compiles successfully, and the daemon process can be managed using dhcpd_start & dhcpd_stop.
The local test results are as follows:

nsh> dhcpd_start eth0
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069568 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 hpwork 0x400cb020 0x400cb0a0
    2     0     0 100 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 lpwork 0x400cb0e0 0x400cb160
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067488 nsh_main
    5     5     4 100 FIFO     Task      - Waiting  Semaphore 0000000000000000 0067464 DHCPD_daemon eth0
nsh> dhcpd_stop eth0
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069568 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 hpwork 0x400cb020 0x400cb0a0
    2     0     0 100 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 lpwork 0x400cb0e0 0x400cb160
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067488 nsh_main
nsh>