tools: moveconfig: simplify show_failed_boards() and show more info

Since commit 1d085568 ("tools: moveconfig: display log atomically
in more readable format"), the function color_text() is clever enough
to exclude LF from escape sequences.  Exploit it for removing the
"for" loops from Slots.show_failed_boards().

Also, display "(the list has been saved in moveconfig.failed)" if
there are failed boards.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>