checkpatch: treat jenkins as valid email

This is needed to avoid checkpatch barking on already committed patches.

This is the old commit cadd51971535 ("checkpatch: treat jenkins as
valid email") re-applied.

Remove the flag BAD_SIGN_OFF, not anymore needed.

Change-Id: I6744f80de982f7934f3a5197ac2df1c29962cbd0
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/5120
Tested-by: jenkins