- Normally, Kontron modules with VPD data stored on embedded KEU
  EEPROM set the 'serial#' environment variable to the value found
  in the 'Serial Number' entry of the SMBIOS block type 2.
- However, on some i.MX8 based modules, the NXP fastboot implementation
  sets 'serial#' in the 'fastboot_setup' function _before_ KEU VPD data
  EEPROM is read. As serial# is write once, it cannot be overridden by
  the value found in VPD data.
- So introduce the new 'board_serial' environment variable that to
  store the serial number from VPD data area. Board specific 'getSerNo'
  function must be adapted to read 'board_serial' from environment
  instead of 'serial#'.
- Keep also to setting the 'serial#' variable as before to be backward
  compatible and to have the 'serial#' variable still set in case that
  FASTBOOT might not have been configured in the bootloader.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>