init: fall back to busybox's shell

Run busybox's shell if AOSP's shell fails to run. This makes
debug shell more reliable.
diff --git a/initrd/init b/initrd/init
index fd2da25..e8899a7 100755
--- a/initrd/init
+++ b/initrd/init
@@ -135,6 +135,7 @@
 	if [ -x system/bin/sh ]; then
 		echo Running MirBSD Korn Shell...
 		USER="($1)" system/bin/sh -l 2>&1
+		[ $? -ne 0 ] && /bin/sh 2>&1
 	else
 		echo Running busybox ash...
 		sh 2>&1