dual-image: Add alt-rwfs-dir config

Add alt-rwfs-dir in meson config to specify the alt-rwfs dir, and set to
ALT_RWFS in config.h

Then sync-manager will sync files to expected directory for the
secondary flash.

Tested: Verify the /etc/hostname is synced to
        /run/media/rwfs-alt/cow/etc/hostname when it is changed.
        Note: the sync-manager only sync files when it's updated.
        Ideally it should sync files once before the service starts to
        make sure the files are always in sync.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I4aee549d339d17529d17e1ec2d9f100ec6b36f07
2 files changed