lp ticker - fix until methods - time is in period addition

[0xc0170]

We should check if now and until are valid compare to the current time.
It can happen that real_now is far in the future, we should not sleep or set interrupt,
just return.

K64F is using deltas for setting the interrupt, we should use real_now
delta = time - real_now

This depends on ARMmbed/mbed-hal#78 (the doc update to clarify this is the intended behaviour).

I wrote simple test cases for this which are now passing with this patch set

void sleep_until_passed()
{
    uint32_t start = lp_ticker_read();
    uint32_t compare_match_start = lp_ticker_get_compare_match();
    // test when now < until < real_now (real_now could wrap around, should not affect it)
    lp_ticker_sleep_until(start - 10, start - 5);
    // we should not set a new compare match
    TEST_ASSERT_EQUAL(compare_match_start, lp_ticker_get_compare_match());
}

void set_interrupt_until_passed()
{
    uint32_t start = lp_ticker_read();
    uint32_t compare_match_start = lp_ticker_get_compare_match();
    // test when now < until < real_now
    lp_ticker_set_interrupt(start - 10, start - 5);

    // compare match should not change
    TEST_ASSERT_EQUAL(compare_match_start, lp_ticker_get_compare_match());
}

This is not ready for merging - one more thing left at least, is time_is_in_period should be available for our targets within HAL as it's intended to be checked. I am proposing to place it in mbed-hal as lp_ticker_time_is_in_period.

@bogdanm @autopulated