build_scripts/layout.ld: add a relro section

This accomadates situations for example: where global variables are
initialized to a value that requires relocation. Things like globally
stored function pointers, or storing the address of another global
variable. Such items require runtime initialization in the form of
dynamic relocation, and cannot be placed in a RO segment. However,
as it is declared to be a constant (not modified by the program), the
dynamic linker can mark it as RO after the dynamic relocation as been
applied [1].

When GCC sees a variable which is constant but requires dynamic
relocation, it puts it into a section named `.data.rel.ro`, further, a
variable that requires dynamic relocation against a local symbol is put
into a `.data.rel.ro.local` section, this helps group such variables
together so that the dynamic linker may apply the relocations, which
will always be RELATIVE locations [1].

[1] https://www.airs.com/blog/archives/189

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>

build_scripts/libtock_layout.ld

@@ -114,6 +114,11 @@ SECTIONS {
     /* Read-only data section. Contains strings and other global constants. */
     .rodata ALIGN(4) : {
         *(.rodata.*)
+
+        /* Generated by GCC in libraries for variables that still have relocations but are constant
+         * at runtime. See https://www.airs.com/blog/archives/189 for background.
+         */
+        *(.data.rel.ro.*)
     } > FLASH
 
     /* Sections located in RAM at runtime.