fs_mg: allow getting VBMeta Digest from device-tree

fs_mgr gets androidboot.vbmeta.{size, hash_alg, digest} from kernel
cmdline to assure the integrity of all vbmeta structs. This CL Allows
fs_mgr to get those parameters from device tree because they're only
consumed by userland instead of kernel.

  https://android.googlesource.com/platform/external/avb/+/master#The-VBMeta-Digest

Bootloader could construct a device tree overlay at run time then merge
it into main device tree. e.g.,

    firmware {
        android {
            vbmeta.size = "5245";
            vbmeta.hash_alg = "sha256";
            vbmeta.digest = "0c51233ca3ecaa...63c6d912e79b709";
        };
    };

Bug: 80168311
Test: boot a device using AVB

Change-Id: I6cf151713af04e6cf554d593e9f0b43e9e214d8c