Makefile: simplify MINIKCONF rules

There is no reason to write MINIKCONF_DEPS manually, since minikconf.py
emits a dependency file, and also no reason to list multiple Kconfig
files on the command line since they can be included from a master file
in the top-level source directory.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/Kconfig b/Kconfig
new file mode 100644
index 0000000..bf694c4
--- /dev/null
+++ b/Kconfig
@@ -0,0 +1,4 @@
+source Kconfig.host
+source backends/Kconfig
+source accel/Kconfig
+source hw/Kconfig