fastboot: fix set_active for legacy A/B.

Legacy A/B implementations use _a and _b slot names, but currently the
set_active command is unconditionally stripping the _ prefix. This CL
adds some code to add the _ back in if connected to a legacy A/B
implementation.

This isn't a complete fix (there are other places in the code that also
unconditionally strip the prefix) but is sufficient for Android Things
use case, and we want to keep this change as small and safe as possible
to avoid breaking anything else.

Bug: http://b/32996227
Test: `fastboot set_active _a` and `fastboot set_active a` works on
      both AndroidThings Edison board and Marlin.
Test: AndroidThings Edison flashall script now completes successfully.
Change-Id: I1a07e48b9e2726f386e9ece5267a9f1d8edefe22