I see that there is a MX25 Flash chip on the Thunderboard React Kit (and that is the same flash chip referred in flashpwr.c/h). Do you have drivers or samples set up with this chip? All I see is the chip being initialized and then powered down.
In case of Simplicity Studio v3 it is in:
In case of Simplicty Studio v4 it is in:
It is a low level driver so please read the flash datasheet before using it!