[zedboot] add unbind

This tool simplifies storage debugging from Zedboot.

Test: manual
Change-Id: I7ffe2fa155f032baaa2393b947535d5d982c23c1
diff --git a/images/zedboot/BUILD.gn b/images/zedboot/BUILD.gn
index 3201415..330f779 100644
--- a/images/zedboot/BUILD.gn
+++ b/images/zedboot/BUILD.gn
@@ -66,6 +66,7 @@
   "bin/ps",
   "bin/sh",
   "bin/umount",
+  "bin/unbind",
 ]
 
 generate_manifest("zedboot.manifest") {