scripts/functions: fix debug-shell

Properly catch resuming the build when continuing past the
failed command.

The 'case ;;&' construct is a bash4ism. Get rid of it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
(transplanted from 2af20cfd210bac338ec18d774458c84fd585ef07)