Unify M5StickC firmware images

[gkoh]

For the M5StickC variants, the primary difference is screen resolution, which is currently compile time defined.
When we migrated to M5Unified, we gained the ability to determine this at runtime.

We should modify the M5ez code to determine the board in ::begin() and set all resolution dependent parameters.
This will remove the need to distinguish between M5StickC hardware variants.

This is also possible for M5Stack Core and Core2, however the flash size is different. For now, I will leave them separate.