Clarify description of rtc_isenabled() and standardize behavior on CI targets

[multiplemonomials]

Summary of changes

My previous PR, #502, identified some issues with the RTC driver's isenabled() method and added a new test. However, turns out I kinda half-assed it: the test was failing on the targets that I have set up for CI, and I also didn't properly update the documentation.

This PR fixes that problem, at least for the targets I have ready access to. I updated the documentation to better explain the new behavior and updated the target code to fix all the test failures.

Note that there may still be some targets with inconsistent behavior of this function, but I expect to catch those as more targets are added to the CI tests. Also, this function's behavior was never very consistent to begin with, so I am not really sure if this is a behavior "change" as much as a behavior "clarification".

Impact of changes

• Updated defined behavior of rtc_isenabled()
• On Freescale Kinetis MCUs, STM32 MCUs, and LPC1768, the rtc_isenabled() function now conforms to the new defined behavior instead of (in most cases) only returning true once the RTC time has been set on the current boot

Migration actions required

If you were using RealTimeClock::isenabled() to detect whether the time had been set yet on the current boot, you will need to instead track that with a flag at the application level. (the behavior of this function was not consistent across targets but on many targets it used to have that behavior)

Documentation

Code docs updated

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] 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

Ran the RTC tests on my six CI boards and they all pass now!

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[JohnK1987]

Defined behaviour says: “returns 1 if the RTC has been initialized and 0 otherwise.”
Function says: “retval 1 The RTC has been initialized and is counting.”