Add libxbc for bootloader bootconfig support am: db746bf101

Original change: https://android-review.googlesource.com/c/platform/bootable/libbootloader/+/1646463

Change-Id: I79d7e9640f34dbdfae84be1ebff0571d5fa81cc0
tree: 9ca17f2be17f89776417c94d9a6d570140c06957
  1. libxbc/
  2. vts/
  3. OWNERS