backuptool: Fix backup/restore functionality

Backup/restore functionality was broken in the
Ia1f4ae95c9e4dae4df844853e81c264bc838f177 change
because of incorrect check of the function's result.

check_prereq() function refactored to return 0 if
backuping/restoration is possible. Any work should be
performed only if check_prereq() succeeds.

Change-Id: Ic977dba675df58a228ef4b882b25beb66cc9d2c6