examples/calib_udelay: Allow the number of delay loop iterations to be specified by kconfig.

[g2gps]

Summary

The calibration test can take a long time on platforms with less processing power. Allow the default value to be changed with kconfig.

Impact

The calibration test completes in a more reasonable time.

Testing

Before this change, with the default 100000 iterations:

nsh> calib_udelay

Calibrating timer for main calibration...
Performing main calibration for udelay. This will take approx. 220.677 seconds.

With this change, and CONFIG_EXAMPLES_CALIB_UDELAY_DELAY_TEST_ITERATIONS=5000

nsh> calib_udelay

Calibrating timer for main calibration...
Performing main calibration for udelay.This will take approx. 12.739 seconds.
Calibration slope for udelay:
  Y = m*X + b, where
    X is loop iterations,
    Y is time in nanoseconds,
    b is base overhead,
    m is nanoseconds per loop iteration.

  m = 127.01355925 nsec/iter
  b = 103458.24360897 nsec

  Correlation coefficient, R² = 1.0000

Without overhead, 0.00787317 iterations per nanosecond and 7873.18 iterations per millisecond.

Recommended setting for CONFIG_BOARD_LOOPSPERMSEC:
   CONFIG_BOARD_LOOPSPERMSEC=7874