[fai] use // operator instead of 'defined .. and' construct (802d5bc)

[Thomas Lange]

>>>>> On Fri, 14 Nov 2014 11:51:11 -0800, Mikhail Lesin <notifications at github.com> said:

    > At least reinstall flag now operates incorrectly, // operator returns contents of $flag_initial, which is also 1 instead of
    > 0 (it was old behavior)

Ooop, this was really a bad change. I fixed this in
adb8ce8ec48a5564bfa0744ac672.  I like to have opinions if this first
change is a important bug. Currently we do not have a bug report for
this. Do we need to fix it for jessie? I guess the next official FAI
version will no make it into jessie, because if has more changes than
bug fixes only.

-- 
regards Thomas