cmd/xb: accept any golang or debian images as bases
cmd/xb: accept any golang or debian images as bases

Change-Id: Id2250ee095f1b3d967c25899a240eaa39d62fe39
Reviewed-on: https://go-review.googlesource.com/c/build/+/173358
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/build,golang/build,golang/build,golang/build