chip/stm32: Save STAY_IN_RO flag if set during reset
chip/stm32: Save STAY_IN_RO flag if set during reset

This matches the behavior of system_encode_save_flags() and allows us to
reboot into RO for testing.

BRANCH=none
BUG=b:156401765
TEST=make buildall -j

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: Ibbb95ca81fb87eaa48639dea99be1bd0e35ea230
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2197621
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec