samples: net: lwm2m_client: Use Ethernet in QEMU Cortex-M3

By default, qemu_coxtex_m3 target selected SLIP driver as
a network backend. This requires some unstandard tool (tunslip6),
which are error prone and might not necessary work.
Ethernet backend work equally well as in native_posix so
use the same for both.

Change the default emulation platform from X86 to Cortex-M3
because it supports Ethernet by default.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>