distro/defaultsetup: Enable security flags by default

distro/defaultsetup: Enable security flags by default

This defaults to including our security flags which use stack-protector-strong
and D_FORTIFY_SOURCE=2 by default, as aids to improve detection of security issues.

This change has been tested in poky for a long time and allows us to align
our default compilation flags and environment.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>