Upload: Fix mbed_adjust_upload_debug_commands for multiple targets

[ccli8]

Summary of changes

The purpose of mbed_adjust_upload_debug_commands() is to adjust first version of MBED_UPLOAD_LAUNCH_COMMANDS. For multiple executalbe cmake targets, MBED_UPLOAD_LAUNCH_COMMANDS becomes not fresh and may cause error. To overcome this, one new cmake cache variable MBED_UPLOAD_LAUNCH_COMMANDS_BAK is introduced to save first version of MBED_UPLOAD_LAUNCH_COMMANDS.

Fix #383

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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[JojoS62]

tested with adding a 3rd target, works as expected. Thanks for the quick fix.