nios2: add _image_rodata_start/end to linker script

This adds the _image_rodata_start and _image_rodata_end
symbols to NIOS2's linker script.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

include/arch/nios2/linker.ld

@@ -133,6 +133,8 @@ SECTIONS
         PROVIDE(gp = _gp);
 #endif
 
+    _image_rodata_start = .;
+
 #include <linker/common-rom.ld>
 
     SECTION_PROLOGUE(_RODATA_SECTION_NAME,,)
@@ -153,6 +155,9 @@ SECTIONS
 
 #include <linker/cplusplus-rom.ld>
 
+    _image_rodata_end = .;
+    _image_rodata_size = _image_rodata_end - _image_rodata_start;
+
     _image_rom_end = .;
     __data_rom_start = ALIGN(4);    /* XIP imaged DATA ROM start addr */