arm: kernel: Parse samsung.board_rev boot option

* Between LL and MM bootloader changes, the name of the boot
  command line option used to pass the board revision changed
  and failure to set the system_rev variable in the kernel due
  to a failure to find the string in the command line would
  cause issues with kernel drivers, the most prominent issue being
  the failure to enter deep sleep.

* Allow for compatibility with all bootloader versions by checking
  for both command line strings (samsung.board_rev for LP and below
  and androidboot.revision for MM and above) and set the system_rev
  variable as necessary.