config: Rename TEXT_SECTION_OFFSET to ROM_START_OFFSET

The `TEXT_SECTION_OFFSET` symbol is used to specify the offset between
the beginning of the ROM area and the address of the first ROM section.

This commit renames `TEXT_SECTION_OFFSET` to `ROM_START_OFFSET` because
the first ROM section is not always the `.text` section.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>