Add a -l option to display a list of offending boards against each
error/warning line. The information will be shown in brackets as below:

02: wip
   sandbox: +   sandbox
       arm: +   seaboard
+(sandbox) arch/sandbox/cpu/cpu.c: In function 'timer_get_us':
+(sandbox) arch/sandbox/cpu/cpu.c:40:9: warning: unused variable 'i' [-Wunused-variable]
+(seaboard) board/nvidia/seaboard/seaboard.c: In function 'pin_mux_mmc':
+(seaboard) board/nvidia/seaboard/seaboard.c:36:9: warning: unused variable 'fred' [-Wunused-variable]
+(seaboard)      int fred;
+(seaboard)          ^

Signed-off-by: Simon Glass <sjg@chromium.org>