init: support early_mount with vboot 2.0 (external/avb/libavb)

libavb requires verifying AVB metadata on all verified partitions at
once. For example, /vbmeta, /boot, /system and /vendor. We need to
invoke device_init() for those partitions even if we only want to early
mount some of them, like /vendor and /system.

This CL gets all AVB partitions and the early mount partitions from
device tree through "firmware/android/vbmeta" and "firmware/fstab",
respectively. The following is an example to early mount /vendor
partition on bullhead:

firmware {
    android {
        compatible = "android,firmware";
        vbmeta {
            compatible = "android,vbmeta";
            parts = "boot,system,vendor";
            by_name_prefix="/dev/block/platform/soc.0/f9824900.sdhci/by-name"
        };
        fstab {
            compatible = "android,fstab";
            vendor {
                compatible = "android,vendor";
                dev = "/dev/block/platform/soc.0/f9824900.sdhci/by-name/vendor";
                type = "ext4";
                mnt_flags = "ro,barrier=1,inode_readahead_blks=8";
                fsmgr_flags = "wait,avb";
            };
        };
    };
};

Bug: 33254008
Test: early mount /vendor with vboot 2.0 (AVB) on bullhead
Test: early mount /system without dm-verity on bullhead
Test: early mount /vendor with vboot 1.0 on sailfish

Change-Id: I89a1f77c97124f309346b33d9e700544b92ecf05