dropbear: pass SFTPSERVER_PATH explicitly

dropbear: pass SFTPSERVER_PATH explicitly

The default value of SFTPSERVER_PATH is "/usr/libexec/sftp-server" defined in
dropbear-2013.58/option.h, but after commit 406bd38b423[bitbake.conf: change
libexecdir to ${libdir}/${BPN}], sftp-server is provided by openssh package,
and is installed into ${libdir}/openssh, so we pass it explicitly.

(From OE-Core rev: 5f6deb044226885912214532cebb1d871f03c53a)

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>