SPL: EXYNOS: Prepare for variable size SPL support

SPL: EXYNOS: Prepare for variable size SPL support

When variable size SPL is used, the BL1 expects the SPL to be
encapsulated differently: instead of putting the checksum at a fixed
offset in the SPL blob, prepend the blob with a header including the
size and the checksum.

The enhancements include
	- adding a command line option, '--vs' to indicate the need for the
	variable size encapsulation
	- padding the fixed size encapsulated blob with 0xff instead of random
	memory contents
	- do not silently truncate the input file, report error instead
	- no need to explicitly closing files/freeing memory, this all happens
	on exit; removing cleanups it makes code clearer
	- profuse commenting
	- modify Makefile to allow enabling the new feature per board

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Rajeshwari S Shinde <rajeshwari.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>