implement boot -d for arm64.

could be done at the end of initarm() except it hangs currently.
this gets you a db> prompt before devices are configured.


Index: evbarm/evbarm/autoconf.c
===================================================================
RCS file: /cvsroot/src/sys/arch/evbarm/evbarm/autoconf.c,v
retrieving revision 1.22
diff -p -u -r1.22 autoconf.c
--- evbarm/evbarm/autoconf.c	27 Aug 2018 09:52:16 -0000	1.22
+++ evbarm/evbarm/autoconf.c	19 Dec 2020 11:16:19 -0000
@@ -171,6 +171,13 @@ cpu_configure(void)
 	struct mainbus_attach_args maa;
 	struct cfdata *cf;
 
+#ifdef DDB
+	if (boothowto & RB_KDB) {
+		printf("Entering DDB...\n");
+		cpu_Debugger();
+	}
+#endif
+
 	(void) splhigh();
 
 	for (cf = &cfdata[0]; cf->cf_name; cf++) {