target: msm8996: dynamical creation for splash image structure

Current splash image header structure is created statically for
splash case, while only works for eMMC device whose blocksize
is 512 bytes. UFS device will fail to boot up when using this
reservation to load splash image. So this patch is to use
dynamical creation for splash image header structure to support
early splash on UFS and eMMC device respectively.

Change-Id: Ifc9f72fe5130a156d62ec922bb3aa4f84656c6c9
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>