configure: Remove old *-config-devices.mak.d files when running configure

configure: Remove old *-config-devices.mak.d files when running configure

When running "make" in a build directory from the pre-Kconfig merge time,
the build process currently fails with:

 make: *** No rule to make target `.../default-configs/pci.mak',
  needed by `aarch64-softmmu/config-devices.mak'.  Stop.

To make sure that this problem at least goes away when the user runs
"configure" (or "sh config.status") again, we have to make sure that
we re-generate the .mak.d files. Thus remove the old stale files
while running the configure script.

Message-Id: <1552300145-12526-1-git-send-email-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>