Fix LwM2M object firmware pull block transfer mode to select size via interface type (BT, ethernet, etc)

[zephyrbot]

Reported by Michael Scott:

Currently, the LwM2M client uses a CONFIG_NET_L2_BLUETOOTH check to determine the block transfer size (64 bytes vs 256 bytes). This should be done via runtime check of the network interface type. There are no good examples of this in Zephyr that I could find, so more research is needed.

(Imported from Jira ZEP-2416)

[nashif]

@mike-scott was this ever fixed? still an issue?

[nashif]

@mike-scott ping

[mike-scott]

@nashif closing some of these issues which will be affected by the move to sockets. We can re-open later if needed.