common-boot: Raise the timeout for flashing images to 60 minutes

Guillaume Tucker recently fixed the LAVA-side meaning of the sd-card device id
to actually point to the SD card on the ARM64 Renesas R-Car m3ulcb boards.

This solves some out-of-space errors but also means that flashing images takes
more time and sometimes 20 minutes are not enough. Raising the timeout to 60
minutes should be plenty.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>