sd-boot: Add a 0.1 second delay before key-probing for showing menu

sd-boot: Add a 0.1 second delay before key-probing for showing menu

If there is no boot menu timeout, pressing a key during boot should get
the boot menu displayed. However, on some systems the keyboard is not
initialized right away, which causes the menu to be inaccessible if no
timeout is specified.

To resolve this, if the error is "not ready" after the initial attempt of
detection, wait for 0.1 second and retry. This solves the problem
described above on all the tested systems.

The reason for just a single retry, and not retrying while "not ready",
is that some firmwares continue to return the "not ready" error on
every probe attempt if no key is pressed.

Signed-off-by: Leonid Bloch <lb.workbox@gmail.com>