security hardening: add RELFO, FORTIFY options

security hardening: add RELFO, FORTIFY options

This enables a user to build a complete system using these
options.  It is important to note that not all packages will
build correctly to start with.

Modeled after OpenWRT approach
https://github.com/openwrt/openwrt/blob/master/config/Config-build.in#L176

A good testing tool to check a target's elf files for compliance
to an array of hardening techniques can be found here:
https://github.com/slimm609/checksec.sh

[Peter: reword fortify help texts, glibc comment]
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>