iot2050: Allow for more than 1 USB storage device

This was lost in refactoring while some users of the IOT2050 expect it
to work: Make sure that up to 3 USB storage devices are probed.

Fixes: 53873974 ("include: armv7: Enable distroboot across all configs")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>