If CONFIG_BOOT_DEV isn't set or otherwise fails to mount,
we prompt the user to change the /boot device, but never
attempt to mount it to ensure it's sane, leading to
potential failures later in the config/boot process.
Ensure that CONFIG_BOOT_DEV is updated after the /boot device
is changed, and attempt to re-mount /boot after the change.
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>