docker: add special rule for deboostrapped images

We might as well have a custom rule for this. For one thing the
dependencies are different. As the primary dependency for
docker-image-% could never be docker-image-debian-bootstrap we can
drop that test in the main rule as well.

Missing EXECUTABLE, DEB_ARCH and DEB_TYPE are treated as hard faults
now. We also error out if the EXECUTABLE file isn't there. We should
really do this with a dependency on any source rules but currently
subdir-FOO-linux-user isn't enough on a clean build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>