samples: blink_led: cleanups and fixups

Align this sample's source code and documentation with other basic
samples.

Adjust the logging configuration and output some additional printk()s
at startup to make it more obvious that initial errors are device
specific and related to calibration.

Fix the period calculation logic so the sample does not stick at the
maximum and minimum periods for two consecutive four-second periods.
That is, instead of setting the pin period to MIN_PERIOD_USEC twice in
a row, do it only once before doubling again, and similarly for
max_period.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>