Tweak upload method dependencies to be specified at a higher level, add LinkServer version check

[multiplemonomials]

Summary of changes

As part of mcuboot support, we need a way to generate upload targets for things that aren't actual CMake build targets. We can alllllmost do that with the existing code, but not quite! Needed to move the add_dependencies() call to the upper level function.

I also noticed a bug while working on this: if old versions of LinkServer are given a hex file, they just treat it as a bin file and flash the ASCII text of the hex file into memory!!!! It's extremely confusing when it happens, and I added a check to catch this situation.

Impact of changes

Migration actions required

Documentation

None

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Used this branch to successfully upload a bootloader and application to a target! Also verified on my machine that the LinkServer check works.