samples: subsys: nvs fix hard fault when running on stm32 targets

on stm32f091 and stm32g071 nucleo boards
This avoids HARD Fault when reaching the last reboot
Disable the external flash memory of the b_u585i_iot02 target
to end the test properly

Signed-off-by: Francois Ramu <francois.ramu@st.com>