refactor mic/chroot.py:global_mounts

describe more clear logic in this function

Signed-off-by: Gui Chen <gui.chen@intel.com>
refactor chrootdir detecting architecture

Signed-off-by: Gui Chen <gui.chen@intel.com>
move savefs to savefs_before_chroot

Signed-off-by: Gui Chen <gui.chen@intel.com>