mgmt: hawkbit: do not prepend custom controllerId

This commit changes how the controllerId is generated based on device
id, and disentangles the two. The controllerId is what hawkbit uses to
uniquely identify a device, and is not necessarily the same as the
device id, and should be fully customizeable by the user if needed.
Previously, all custom device ids were being prepended with
`CONFIG_BOARD`. When a user selects `CONFIG_HAWKBIT_CUSTOM_DEVICE_ID`,
they should be able to specify the full controllerId used with hawkbit,
without a forced prepend.

Signed-off-by: Neal Jackson <neal@blueirislabs.com>