sanitycheck: Update section names for memory size calculation

This commit updates the section names for memory size calculation in
the sanitycheck script as follows:

1. Remove `_TEXT_SECTION_NAME_2` section, which no longer exists, from
  the `rw_sections`.

2. Add `rom_start` section, which mostly contains read-only data such
  as the exception vector table, to the `ro_sections`.

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