busybox

make_single_applets: fix ": $((fail++))" expansion error

$((fail++)) is not a required expression in POSIX, and in "dash" it
could produce an error like this:

    ./make_single_applets.sh: 61: arithmetic expression: expecting primary: "fail++"

Replace this with something portable: fail=$((fail+1)) would work.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>