improve function set_active_system

Change-Id: I1623abc4e54a04f7c82876cf0ae8fb538b0b2e6a
Partly integrated 699c7263ab25c6ee84870c08f07c6722b81e6248 from M1cha:
- fix: backup/restore boot1
- fix: ask user to select system for advanced restore of boot and system
- optimize: only show existing images in advanced restore menu
Set default storage location to /sdcard/0:

  - clockworkmod data location (e.g. nandroid) is /sdcard/0/clockworkmod
  - default update.zip location is /sdcard/0/update.zip