The RSTOUT# signal cannot be used with the Flash Memory for boot. It is recommended to use the RES# signal and others.
RSTOUT# can be used as a reset signal to other external devices, if the RSTOUT# pin outputs low level for 500 us (typ.) when reset occurs with RES# signal. As boot processing of the RZ/T1 starts after reset releases the RES# pin, there is a possibility that reading from the flash memory may not be ready in time when RSTOUT# pin is used as reset for flash memory.
For more details, refer to RZ/T1 Group User’s Manual: Hardware (R01UH0483) "6.3.6 Reset Output Pin (RSTOUT#)".