So I guess:
- I should set the  external flash region as FLASH.
- Set the embedded flash region as EMB_FLASH or something.
- Use a custom ".embedded-flash" section to put things into the EMB_FLASH region.
- Ensure that the `.vector_table` section is actually put into EMB_FLASH instead of FLASH as cortex-m-rt does by default?
- Mark the entry as ".embedded-flash" linker section attribute.?
- Anything else ?