fs_mgr: consolidate DT compatible check

Fixes the compatible check in fs_mgr_boot_config by consolidating the
check in a single privately exported function within fs_mgr (i.e.
is_dt_compatible()).

b/27805372

Test: Boot sailfish w/ early mount

Change-Id: Ie2d1646b81cf9eba8d16828ca8cf2c75156c294c
Signed-off-by: Sandeep Patil <sspatil@google.com>