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>
diff --git a/Makefile b/Makefile
index af2ab4e..da8a38d 100644
--- a/Makefile
+++ b/Makefile
@@ -796,6 +796,12 @@
 BOARD_SIZE_CHECK =
 endif
 
+ifneq ($(CONFIG_SPL_SIZE_LIMIT),0)
+SPL_SIZE_CHECK = @$(call size_check,$@,$(CONFIG_SPL_SIZE_LIMIT))
+else
+SPL_SIZE_CHECK =
+endif
+
 # Statically apply RELA-style relocations (currently arm64 only)
 # This is useful for arm64 where static relocation needs to be performed on
 # the raw binary, but certain simulators only accept an ELF file (but don't
@@ -1712,6 +1718,8 @@
 
 spl/u-boot-spl.bin: spl/u-boot-spl
 	@:
+	$(SPL_SIZE_CHECK)
+
 spl/u-boot-spl: tools prepare \
 		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb) \
 		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_TPL_OF_PLATDATA),dts/dt.dtb)