drivers: net: adding NET_DRIVERS menuconfig

Fixes: #38403

Adding NET_DRIVERS menuconfig so that network drivers are grouped
together in its own menu entry under drivers, similar to most other
drivers.

This further has the advantages that `CONFIG_NET_DRIVERS` can be used
for testing to determine if network drivers has been selected.

This changed revealed a dependency loop where both `select` (for SLIP)
and `depends` (for PPP) which both depends on NET_DRIVERS` where in use
in the dependency tree for Qemu networking, especially NET_SLIP_TAP.

This is handled by defaulting `NET_DRIVERS` to `y` when building for a
Qemu target.
`SLIP` had a dependency to `!QEMU_TARGET || NET_QEMU_SLIP`. This is
changed so that SLIP prompt depends on `!QEMU_TARGET` which provides
full user control in hardware but makes the symbol promptless on Qemu
targets.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>