init: fstab: add support to read fstab entries from device tree

for early mount, we need a way to tell init where to find vendor,
odm partitions (also system in case of non-A/B devices). Also, that
needs to be independent of kernel cmdline since the cmdline will likely
exceed its limit.

The change adds support for parse and create fstab entries that can be
directly sent to the fs_mgr for mounting partitions early in init first
stage.

Sample DT entry to mount vendor partition early on angler-

firmware {
    android {
        compatible = "android,firmware";
        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";
            };
        };
    };
};

b/27805372

Test: Boot angler and sailfish with early "vendor" partition mount by
adding aforementioned DT node and enable CONFIG_PROC_DEVICETREE in kernel

Change-Id: I669013e3fdb157e88719436534f63989dec95d60
Signed-off-by: Sandeep Patil <sspatil@google.com>