VPP-1341: fix loopback interface graph arcs

VPP-1341: fix loopback interface graph arcs

Remove broken special case from l2_input.c:set_int_l2_mode(), which
turns out to confuse the graph dispatch engine.

The loopback TX function needs to push packets to either
ethernet-input or to l2-input, based on bridge / BVI
configuration. Rather than overloading a single graph arc - and making
vain attempts to reconfigure it - create both arcs and use the correct
one.

Rewrote the loopback tx function as an idosyncratic multi-arch
quad/single loop fn.

Change-Id: I15b56ce641d90a11e7b3c7d23859f40e168dd7b2
Signed-off-by: Dave Barach <dave@barachs.net>