Change Default 'preferred_export_format' and Add Menu Option

[oclyke]

Subject of the issue

The preferred_export_format option is set to axf for all supported boards. This causes an incompatibility with Arduino-CLI as described in #104.

Expected behaviour

Arduino CLI should be useful for compiling and uploading binaries to Apollo3-based boards.

Actual behaviour

Production of a .axf output is incompatible with the Arduino CLI and or upload tools.

Proposed Solution:

Let's change the default option to bin as suggested by @frinkahedron and also provide a menu option so that users may generate an axf file for use with debuggers.

[oclyke]

After looking into solutions the menu option was blocked by the opacity of Arduino's process for exporting the compiled binary. Instead I opted to simply add a hook that copies out the .axf file in addition to the .bin

This will be resolved by #136