first_stage_mount: mount point must be canonical path

Ban weird paths such as /../system or //vendor in first stage mount.
Add utility function fs_mgr_create_canonical_mount_point() that:

* mkdir(mount_point) to ensure mount_point's existence
* Test that realpath(mount_point) =?= mount_point

Bug: 188898525
Test: Presubmit
Test: Boot CF
Change-Id: Iaf2ec52701277f26cc81f3e15a47b6083a788334