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lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 058f:6254 Alcor Micro Corp. USB Hub
Bus 001 Device 007: ID 2341:0042 Arduino SA Mega 2560 R3 (CDC ACM)
Bus 001 Device 005: ID 058f:6366 Alcor Micro Corp. Multi Flash Reader

USB serial adapters

kmod-usb-acm
2341  Arduino SA
    0001  Uno/Uno SMD R2 (CDC ACM)
    0010  Mega 2560 (CDC ACM)
    003b  Serial Adapter (CDC ACM)
    003d  Due Programming (CDC ACM)
    003e  Due Native (CDC ACM)
    003f  Mega ADK (CDC ACM)
    0042  Mega 2560 R3 (CDC ACM)
    0043  Arduino Uno R3 (CDC ACM)
    0044  Arduino Mega ADK R3 (CDC ACM)
...

opkg update 
opkg install kmod-usb-acm

ls -al /dev/ttyACM0
crw-rw-rw-    1 root     root      166,   0 May  6 01:20 /dev/ttyACM0