sandbox: keep time offset when resetting

The UEFI Self Certification Test (SCT) checks the SetTime() service with
the following steps:

* set date
* reset
* check date matches

To be compliant the sandbox should keep the offset to the host RTC during
resets. The implementation uses the environment variable
UBOOT_SB_TIME_OFFSET to persist the offset.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>