Fix relative paths to custom upload method file, fix incorrect CAN length on STMicro

[multiplemonomials]

Summary of changes

Noticed while working on the custom targets example that if you pass a relative path to CUSTOM_UPLOAD_CFG_PATH, it errors out, because EXISTS in CMake only works with absolute paths (seriously this is such a common foot-gun and they still have not fixed it).

Also noticed a compiler warning coming from the STM32 CAN code. It appears that there was some unintentional behavior happening because the < operator has higher precedence than &, so the comparison was happening before the mask! It looks like they were trying to get the length from the DLC field, so I updated the code to use a proper register definition and parentheses as needed.

Impact of changes

Migration actions required

Documentation

None

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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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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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