Partition names are uppercase on galaxy-s2.

The [documentation](https://developer.mozilla.org/en-US/Firefox_OS/Firefox_OS_build_prerequisites#Follow_Samsung_Galaxy_S2_extra_steps) asks users to make this change manually, but the `flash_heimdall()` function is only used by this device so it makes sense to default to the correct names.

Making this more generic will probably be a good idea once more Samsung devices are supported.