Actually /proc is a virtual file system which does not contain 'real' files but virtual ones exposed by the Linux kernel.
/proc/device-tree contains an image of the Open Firmware device tree (DT), a description of the system hardware.
Having a unique BD address is critical, the board is provisioned with addresses in factory and the bootloader which has access to this provisioned data injects the 'local-mac-address' property in the device-tree which is then exported to the Linux kernel.
This is why you can see this property in:
You don't need to change this prop, but only read it (binary) to configure the Bluetooth controller with this unique address. Today you can use btmgmt tool to configure the controller address.
btmgmt public-addr 12:34:56:78:9A:BC
However we are currently working on a solution to integrate this in the driver in order to avoid relying on user/user-space to configure the Bluetooth address.