Install nmap:

opkg update
opkg install nmap

Scan local network:

nmap -sP 192.168.0.0/24
Starting Nmap 6.01 ( http://nmap.org ) at 2015-08-05 12:10 UTC
Nmap scan report for 192.168.0.1
Host is up (0.0016s latency).
MAC Address: 14:CF:E2:A2:42:A7 (Unknown)
Nmap scan report for 192.168.0.20
Host is up (0.00022s latency).
MAC Address: 00:1E:C9:30:2E:68 (Dell)
Nmap scan report for 192.168.0.100
...