configs: define CONFIG_SPL_SIZE_LIMIT

configs: define CONFIG_SPL_SIZE_LIMIT

A new configuration variable CONFIG_SPL_SIZE_LIMIT is introduced to define
the board specific maximum size for the SPL file.

Use Makefile function size_check() to implement the test.

Depending on the size of CONFIG_SPL_SIZE_LIMIT an error like the following
is thrown:

spl/u-boot-spl.bin exceeds file size limit:
  limit:  30720 bytes
  actual: 33426 bytes
  excess: 2706 bytes
make: *** [Makefile:1663: spl/u-boot-spl.bin] Error 1

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>