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>

Config.in

@@ -734,6 +734,76 @@ endchoice
 comment "Stack Smashing Protection needs a toolchain w/ SSP"
 	depends on !BR2_TOOLCHAIN_HAS_SSP
 
+choice
+	bool "RELRO Protection"
+	depends on BR2_SHARED_LIBS
+	help
+	  Enable a link-time protection know as RELRO (RELocation Read Only)
+	  which helps to protect from certain type of exploitation techniques
+	  altering the content of some ELF sections.
+
+config BR2_RELRO_NONE
+	bool "None"
+	help
+	  Disables Relocation link-time protections.
+
+config BR2_RELRO_PARTIAL
+	bool "Partial"
+	help
+	  This option makes the dynamic section not writeable after
+	  initialization (with almost no performance penalty).
+
+config BR2_RELRO_FULL
+	bool "Full"
+	help
+	  This option includes the partial configuration, but also
+	  marks the GOT as read-only at the cost of initialization time
+	  during program loading, i.e every time an executable is started.
+
+endchoice
+
+comment "RELocation Read Only (RELRO) needs shared libraries"
+	depends on !BR2_SHARED_LIBS
+
+choice
+	bool "Buffer-overflow Detection (FORTIFY_SOURCE)"
+	depends on BR2_TOOLCHAIN_USES_GLIBC
+	depends on !BR2_OPTIMIZE_0
+	help
+	  Enable the _FORTIFY_SOURCE macro which introduces additional
+	  checks to detect buffer-overflows in the following standard library
+	  functions: memcpy, mempcpy, memmove, memset, strcpy, stpcpy,
+	  strncpy, strcat, strncat, sprintf, vsprintf, snprintf, vsnprintf,
+	  gets.
+
+	  NOTE: This feature requires an optimization level of s/1/2/3/g
+
+	  Support for this feature has been present since GCC 4.x.
+
+config BR2_FORTIFY_SOURCE_NONE
+	bool "None"
+	help
+	  Disables additional checks to detect buffer-overflows.
+
+config BR2_FORTIFY_SOURCE_1
+	bool "Conservative"
+	help
+	  This option sets _FORTIFY_SOURCE to 1 and only introduces
+	  checks that shouldn't change the behavior of conforming
+	  programs.  Adds checks at compile-time only.
+
+config BR2_FORTIFY_SOURCE_2
+	bool "Aggressive"
+	help
+	  This option sets _FORTIFY_SOURCES to 2 and some more
+	  checking is added, but some conforming programs might fail.
+	  Also adds checks at run-time (detected buffer overflow
+	  terminates the program)
+
+endchoice
+
+comment "Fortify Source needs a glibc toolchain and optimization"
+	depends on (!BR2_TOOLCHAIN_USES_GLIBC || BR2_OPTIMIZE_0)
 endmenu
 
 source "toolchain/Config.in"