linker: align _image_rodata and _image_rom start/end/size linker symbols

Cleanup and preparation commit for linker script generator.

Zephyr linker scripts provides start and end symbols for each larger
areas in the linker script.

The symbols _image_rom_start and _image_rom_end corresponds to the group
ROMABLE_REGION defined in the ld linker scripts.

The symbols _image_rodata_start and _image_rodata_end is not placed as
independent group but covers common-rom.ld, thread-local-storage.ld,
kobject-rom.ld and snippets-rodata.ld.

This commit align those names and prepares for generation of groups in
linker scripts.

The symbols describing the ROMABLE_REGION will be renamed to:
_image_rom_start -> __rom_region_start
_image_rom_end   -> __rom_region_end

The rodata will also use the group symbol notation as:
_image_rodata_start -> __rodata_region_start
_image_rodata_end   -> __rodata_region_end

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>