net: lwm2m: Reduce dependency on firmware update pull Kconfig

There are functions whose headers are only visible when
CONFIG_LWM2M_FIRMWARE_UPDATE_PULL_SUPPORT is selected. However these
functions are used outside the scope of the firmware update pull
functionality. This change allows to disable
CONFIG_LWM2M_FIRMWARE_UPDATE_PULL_SUPPORT while still being able to use
the firmware update object.

Signed-off-by: Marc Lasch <marc.lasch@husqvarnagroup.com>