xilinx-bsp: Rewrite to handle xilinx virtex4 virtex5 targets

xilinx-bsp: Rewrite to handle xilinx virtex4 virtex5 targets

* Rewrite for handling most xilinx target virtex4 and virtex5 based
* Add helper functions for configuring the target board
* User must add in local.conf:
  XILINX_BOARD = "ml401" | "ml403" | "ml405" | "ml410" | "ml507" | "ml510"
  XILINX_BSP_PATH = "Xilinx XPS  hardware project dir"
  with this variables u-boot and the linux kernel gets configured
  for a selected board.
* xilinx-bsp exits when there is no hardware project or when parameters
  header or the linux kernel device tree haven't been generated in
  Xilinx XPS [1] in order to match hardware/software model
* Only tested for powerpc based hardware projects
* [1] Xilinx XPS -> Xilinx Platform Studio
* v2 add additional comments

Signed-off-by: Adrian Alonso <aalonso00@gmail.com>