dropbear: fix interface config setting

Patch from #15070

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40298 3c298f89-4303-0410-b956-a3cf2f4a3e73
dropbear: add options SSHKeepAlive and IdleTimeout.

Without timeout mechanism, if ssh client disconnected without sending
FIN or RST, forked dropbear servers would hang there for
KEX_RETRY_TIMEOUT seconds (8 hours).

TCP keepalive is not implemented in dropbear yet, thus the name
SSHKeepAlive.

300 seconds in this patch is selected from the default value of
ServerAliveInterval for Debian ssh client (See man ssh_config).

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40299 3c298f89-4303-0410-b956-a3cf2f4a3e73
dropbear: move options.h editing to Build/Configure

fixes incremental build with change to CONFIG_DROPBEAR_ECC
drop --with-shared which is unknown to configure

Patch by Catalin Patulea <cat@vv.carleton.ca>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40300 3c298f89-4303-0410-b956-a3cf2f4a3e73