first stage mount: support mount points like /vendor/abc

Current syntax of the fstab in device tree (fstab_dt) assumes the
node name is the mount point, which doesn't allow subdir:

    vendor {   <== using "vendor/abc" leads to syntax error in device tree
        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";
    };

This CL adds a new field "mnt_point" in the fstab_dt configuration:

    vendor_abc {
        compatible = "android,vendor_abc";
        dev = "/dev/block/platform/soc.0/f9824900.sdhci/by-name/abc";
        type = "ext4";
        mnt_flags = "ro,barrier=1,inode_readahead_blks=8";
        mnt_point = "/vendor/abc";                          <== new field
        fsmgr_flags = "wait";
    };

The new field is optional and will fallback to using node name as mount
point if it is absent.

Note that this CL also sorts fstab_dt by mount point, because
/vendor needs to be mounted first, which contains a directory
/vendor/abc, before /vendor/abc can be mounted.

Bug: 69955336
Test: first stage mount /vendor/abc on a device
Change-Id: Ie2e519f9801f211a7a221622f32c82bedd00353f