brppt1: Update environment to support new boot concept

* Drop legacy /boot/PPTImage.md5 check
* Update device tree naming
* Update t30args#0 root cmd line property to support latest kernel
  versions (root=/dev/mmcblk0p2 for linux < 4 and
  root=/dev/mmcblk1p2 for linux >= 4)
* Add custom bootloader version string
* Destroy invalid dtb at ${dtbaddr} and configuration script at
  ${cfgaddr} to ensure proper boot in warm restart case.

Signed-off-by: Bernhard Messerklinger <bernhard.messerklinger@br-automation.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>