Implement GBL fastboot backend

Implements fastboot backends using generic GBL interfaces. Initially
support `fastboot getvar [partition|block-device]`.

Bug: 328784766
Change-Id: Ic236bb8cefc841e13185fcacccb4b42796fa6ae8
12 files changed
tree: 0c4e9b98cea5b76c38d76e931a3299fa880c46c8
  1. gbl/
  2. libxbc/
  3. vts/
  4. .gitignore
  5. BUILD
  6. OWNERS