mpc5200: Add a4m2k board port

This patch adds the a4m2k MPC5200B board port. Its a derivate of
the a3m071 board with only minor changes.

Additionally this patch includes some clean-up changes:
- Remove I2C support from a3m071 as its unused
- Fix/enhance default env variables
- Fix some comments
- Add newly introduced CONFIG_SPL_TARGET to automatically build
  "u-boot-img.bin"
- Fix dtb patching in READ desciption for SPL Linux booting:
  "fdt chosen" needs to get called to patch/create the chosen node.
- Add missing call to spl_board_init():
  Define CONFIG_SPL_BOARD_INIT so that spl_board_init() will get
  called in the SPL version.

Signed-off-by: Stefan Roese <sr@denx.de>