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>

doc/arch/sandbox.rst

@@ -131,6 +131,13 @@ available options. Some of these are described below:
 -i
   Go to interactive mode after executing the commands specified by -c.
 
+Environment Variables
+---------------------
+
+UBOOT_SB_TIME_OFFSET
+    This environment variable stores the offset of the emulated real time clock
+    to the host's real time clock in seconds. The offset defaults to zero.
+
 Memory Emulation
 ----------------