Add new custom command 'rstbywdt' for last reset reason check

Add new custom command 'rstbywdt' for last reset reason check

Add a small helper command 'rstbywdt' which returns true if the last
system reset was caused by the SOC watchdog.

It can be used for example to boot different kernel in case of reset
caused by watchdog:

if rstbywdt; then bootm 0x9FE80000; else bootm 0x9F050000; fi

Also, show a warning at boot if last reset was caused by watchodg.