arm: define _image_rodata_start/end

Define _image_rodata_start/end to match x86 and so that we can
refer to them in the userspace test among others.

Signed-off-by: Stephen Smalley <[email protected]>

Author: stephensmalley

include/arch/arm/cortex_m/scripts/linker.ld

@@ -139,6 +139,8 @@ SECTIONS
 	__exidx_end = .;
 	} GROUP_LINK_IN(ROMABLE_REGION)
 
+	_image_rodata_start = .;
+
 #include <linker/common-rom.ld>
 
     SECTION_PROLOGUE(_RODATA_SECTION_NAME,,)
@@ -167,6 +169,7 @@ SECTIONS
 	. = ALIGN(4);
 	} GROUP_LINK_IN(ROMABLE_REGION)
 
+	_image_rodata_end = .;
 	_image_rom_end = .;
 
     GROUP_END(ROMABLE_REGION)