net: Restore the default networking behavior on qemu platforms

Commit e99b5228 changed the default
networking behavior on qemu platforms, as the SLIP_TAP driver no longer
selected Ethernet L2. In results, qemu platforms in default
configuration had no network interface to work with.

Fix this, by restoring select for NET_L2_ETHERNET, but this time in
NET_SLIP_TAP. The difference is that SLIP_TAP is a generic driver
config, which could be enabled on any platform, so not changing the
behavior here, while NET_SLIP_TAP is a config dedicated for qemu only,
so it's reasonable to enable all that's needed to get SLIP to work with
qemu by default.

As there were circular Kconfig dependencies, move the respective qemu
configs outside of NET_RAW_MODE if block (as this caused the circular
dependency with ETH_DRIVER_RAW_MODE). Also, remove dependency to
NET_L2_BT, as it made little sense there, and also caused dependency
problems.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>