discover: Perform pre-boot steps when timeout expires

discover: Perform pre-boot steps when timeout expires

When a default boot timeout expires boot() is called via
default_timeout() rather than device_handler_boot(). default_timeout()
doesn't call platform_pre_boot() beforehand, which means steps such as
clearing a temporary boot device override are skipped.
Add a call to platform_pre_boot() immediately before boot() to ensure
these steps are performed regardless of boot type.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>