Fix `flash_offset` collision in mix.exs

[UncleGrumpy]

Changes the mix.exs flash_offset option to include the platform name, to allow configuring an application for both esp32 and stm32 platforms in the mix.exs file. The original name is retained for command line over-rides.

closes #30

[joustava]

This has been approved already but maybe good to add some comment explaining the difference in flash_address between the elixir and non-elixir images?

[UncleGrumpy]

This has been approved already but maybe good to add some comment explaining the difference in flash_address between the elixir and non-elixir images?

Other than being more explicit about needing to use an elixir support image, I think any reference to the offset used in the Erlang only images would lead to more confusion.