Fix a few CMake issues after the recent upload and memory bank changes

[multiplemonomials]

Summary of changes

• I renamed the GDB_PORT variable but forgot to do a usage search 🤦 🤦 🤦
• issue Need help on memory_banks.json for custom targets #368 : building without defined memory banks caused a build error because memap.py was looking for memory_banks.json even if it didn't exist

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

(though maybe we need to have some actual testing of the debug configurations, that's how this got in in the first place...)