MFC r283547:

MFC r283547:

  Ensure that all arm kernel configs contain ALT_BREAK_TO_DEBUGGER and not
  BREAK_TO_DEBUGGER if they have a serial console (most do).  A burst of
  serial line noise (such as unplugging a usb serial adapter) can look like
  a break and drop a working system into the debugger.  The alt break sequence
  (<CR>~^B) works fine on both serial and non-serial consoles.

git-svn-id: svn://svn.freebsd.org/base/stable/10@287082 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f