obmc-flash-bmc: write variables twice to ensure both banks are correct

The U-Boot environment maintains two banks of environment variables. The
banks need to be consistent with each other to ensure that these
variables can be reliably read from file, as they would be in the
context of a C++ program. In order to guarantee that the two banks are
both correct, we need to run fw_setenv twice when writing environment
variables.

This change accompanies a change to phosphor-bmc-code-mgmt for
openbmc/openbmc#2666.

Change-Id: I72d7fd79faf271d017703464fd9ad0cd20f4e2a0
Signed-off-by: Michael Tritz <mtritz@us.ibm.com>