add getting started cross compile

[simon88]

Hi, It would be nice to add a getting started, explain how to build PSDK without specific IDE like Keil MDK, just with cmake/make for a board.

It would be nice to add Cmake file for bootloader app not only application

[dji-dev]

Agent comment from Leon in Zendesk ticket #100866:

Dear Developer,

Hello, thank you for contacting DJI.

Thank you for your feedback and suggestions. We will synchronize this suggestion to the internal team and ask them to evaluate and supplement the relevant guidance documents.

Thank you for your support of DJI products.

Best Regards, DJI Dajiang innovation SDK technical support

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