dropbear: add option to set receive window size

dropbear: add option to set receive window size

The default receive window size in dropbear is hardcoded to 24576 byte
to limit memory usage. This value was chosen for 100Mbps networks, and
limits the throughput of scp on faster networks. It also severely limits
scp throughput on high-latency links.

Add an option to set the receive window size so that people can improve
performance without having to recompile dropbear.

Setting the window size to the highest value supported by dropbear
improves throughput from my build machine to an APU2 on the same LAN
from 7MB/s to 7.9MB/s, and to an APU2 over a link with ~65ms latency
from 320KB/s to 7.5MB/s.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>