K64F linker script refactor, also fix some misc bugs

[multiplemonomials]

Summary of changes

This MR refactors the K64F linker script to support memory bank configuration. This is needed as I'm using it to test the MCUBoot bootloader.

I also fixed a couple misc bugs I ran into while working on MCUBoot.

Impact of changes

K64F can now be configured to use only a portion of flash using target_overrides.memory_bank_config in mbed_app.json.

Migration actions required

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] 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

I reran the Mbed test suite for K64F. Results here, don't see any suspicious failures.

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[JohnK1987]

This is cool, I like that.

/* If we are not configured to execute out of the start of ROM, then a bootloader is
 * present.  This tells us whether we need to add the Flash Configuration Field at the start of flash. */
#if MBED_CONFIGURED_ROM_BANK_IROM1_START == 0
#define IS_BOOTLOADER_PRESENT 0
#else
#define IS_BOOTLOADER_PRESENT 1
#endif

[JohnK1987]

Unfortunately I don't have much experience with linker script, only a marginal awareness. So I am not able to say anything to these changes :D
BTW when we want to add an information header (version, release date and so on) to application. Is also necessary to modify linker script?

[multiplemonomials]

BTW when we want to add an information header (version, release date and so on) to application. Is also necessary to modify linker script?

If you mean like an mcuboot header, no, mcuboot prepends that to the binary file after it's been linked. If you are rolling your own format, and you just want to include a specific C struct at the start of the image, then yeah, you can do that with a linker script.