QEMU_CFLAGS is not available in pc-bios/s390-ccw/netboot.mak, but the Makefile
needs to access the flags passed to the configure script for the s390x
cross compiler.  Fix everything and rename QEMU_CFLAGS to EXTRA_CFLAGS for
consistency with tests/tcg.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>