run: skip libc_ffat test for non qemu test runs

Print a warning that the sd respectively hard disc must be prepared manually.

Fixes #783
lwip: set to max snd window which is known to work

The window scale option (http://tools.ietf.org/html/rfc1323) patch of lwIP
definitely works solely for the receive window, not for the send window.
Setting the send window size to the maximum of an 16bit value, 65535,
or multiple of it (x * 65536 - 1) results in the same performance.

Everything else decrease performance.

We will have to check this window scale patch before using higher values.
run: add nic_bridge to netperf native test
lwip: use memcpy of Genode