scripts: mkits.sh: Allow legacy @ mode for dts creation

commit 5ec60cbe ("scripts: mkits.sh: replace @ with - in nodes")
broke support for Meraki MR32 and this patch makes the replacement
configurable allowing for specifying the @ or - or whatever character
that is desired to retain backwards compatibility with existing devices.

For example, this patch includes the fix for the Meraki MR32 in
target/linux/bcm53xx/image for meraki_mr32:

  DEVICE_DTS_DELIMITER := @
  DEVICE_DTS_CONFIG := config@1

Fixes: 5ec60cbe ("scripts: mkits.sh: replace @ with - in nodes")
Signed-off-by: Damien Mascord <tusker@tusker.org>
[Added tags, checkpatch.pl fixes, noted that this is for old stuff]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>