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root@Arduino:~# lsusb
...
Bus 001 Device 005: ID 0a5c:21e8 Broadcom Corp. BCM20702A0 Bluetooth 4.0

Install Bluetooth software:

opkg update 
opkg install kmod-bluetooth bluez-libs bluez-utils

Manually mount it:

root@Arduino:~# echo 0a5c 21e8> /sys/bus/usb/drivers/btusb/new_id

root@Arduino:~# hciconfig
hci0:   Type: BR/EDR  Bus: USB
       BD Address: 00:02:72:33:96:0A  ACL MTU: 1021:8  SCO MTU: 64:1
       DOWN
       RX bytes:489 acl:0 sco:0 events:20 errors:0
       TX bytes:86 acl:0 sco:0 commands:20 errors:0

Plugable® USB Bluetooth 4.0 Low Energy Micro Adapter

Amazon Price:$11.95

IOGEAR Bluetooth 4.0 USB Micro Adapter

Amazon Price:$11.77