imx: move BOARD_SIZE_CHECK to main Makefile

We currently have duplicate definitions for BOARD_SIZE_CHECK in Makefile
and arch/arm/mach-imx/Makefile.

Move the board size check from arch/arm/mach-imx/Makefile to Makefile.

Depending on the value of CONFIG_BOARD_SIZE_LIMIT an error like an error
like the following is thrown:

u-boot-dtb.imx exceeds file size limit:
  limit:  503696 bytes
  actual: 509720 bytes
  excess: 6024 bytes
make: *** [Makefile:1051: u-boot-dtb.imx] Error 1

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>