docker: drop QEMU_TARGET check, fallback in EXECUTABLE not set

The addition of QEMU_TARGET was intended to ensure we fall back to
checking for the existence of an image if the build system was not
currently configured to build it. However this breaks the direct use
of the rule for building custom binfmt_misc images. We already check
for EXECUTABLE so let us just use that as a proxy for deciding if we
are just going to check the image exits.

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