bootloader: check previously used clock frequency at run time

bootloader: check previously used clock frequency at run time

In the situation when bootloader was compiled for 240MHz, and app was
compiled for 160MHz, and the chip is a revision 0 chip, the
bootloader will assume that the application has also been running at
240MHz. This will cause the chip to lock up later. Modify this to use
a run time check of DPORT_CPUPERIOD_SEL, which indicates which of the
PLL frequencies was used.

Closes https://github.com/espressif/esp-idf/issues/2731.