Support legacy image for backward-compatibility

Support legacy image for backward-compatibility

Work flows for migration
-> helper daemon will call app2ext_migrate_legacy_all()
-> call pkgmgr client api to execute the installer backend
-> the backend mount legacy image
-> the backend change old labels
-> the backedn umount legacy image

Submit with :
        [slp-pkgmgr] https://review.tizen.org/gerrit/#/c/107123/
        [app-installers] https://review.tizen.org/gerrit/#/c/107124/
        [tpk-backend] https://review.tizen.org/gerrit/#/c/107125/
        [pkgmgr-server] https://review.tizen.org/gerrit/#/c/108346/
        [wgt-backend] https://review.tizen.org/gerrit/#/c/108485/

Migration Trigger Test:
	#test_app2ext --migrate-all (refer to usage)

Change-Id: I180d6144d3fa033f7c969dc76f4f0f3e5b20e91f
Signed-off-by: jongmyeongko <jongmyeong.ko@samsung.com>