In zephyr_linker_sources().
This is done since the point of the location is to place things at given
offsets. This can only be done consistenly if the linker code is placed
into the _first_ section.
All uses of TEXT_START are replaced with ROM_START.
ROM_START is only supported in some arches, as some arches have several
custom sections before text. These don't currently have ROM_START or
TEXT_START available, but that could be added with a bit of refactoring
in their linker script.
No SORT_KEYs are changed.
This also fixes an error introduced when TEXT_START was added, where
TEXT_SECTION_OFFSET was applied to riscv's common linker.ld instead of
to openisa_rv32m1's specific linker.ld.
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>