dropbear: fix build with non-SSP capable toolchain

dropbear: fix build with non-SSP capable toolchain

By default, Dropbear's configure script enables hardening
flags. Unfortunately, the check for SSP only uses AC_COMPILE_IFELSE(),
and therefore doesn't properly test for the availability of libssp,
visible only at link time.

In addition, Buildroot passes its own hardening flags, depending on
various global options. So, we simply disable hardening flags in
Dropbear.

This fixes a build failure with non-SSP capable toolchains happening
since the bump to 2018.76.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>