linker: align _image_text_start/end/size linker symbols name

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_text_start and _image_text_end sometimes includes
linker/kobject-text.ld. This mean there must be both the regular
__text_start and __text_end symbols for the pure text section, as well
as <group>_start and <group>_end symbols.

The symbols describing the text region which covers more than just the
text section itself will thus be changed to:
_image_text_start -> __text_region_start
_image_text_end   -> __text_region_end

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