Makefile: Forbid out-of-tree build from a source tree that has been built in

If we try to do an out-of-tree build but the source tree we're building from
has been used in the past for an in-tree build then things will go
confusingly wrong. Specifically, some parts of the build process will pull
in generated files from the old in-tree build (because SRC_PATH is on
the vpath). Diagnose this situation so we can produce a useful error
message and tell the user how to fix it (run distclean in the source tree).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>