fwu: Initialize global fwu library state during CI test

The current CI test worked by sheer luck, the g_dev global pointer
in the fwu library was never initialized and the test equally well
failed on sandbox64. Trigger the main loop in sandbox tests too to
initialize that global state, and move the sandbox specific exit
from fwu_boottime_checks after g_dev is initialized.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Acked-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>