tools: mkimage: code abstraction generic and image specific

tools: mkimage: code abstraction generic and image specific

This is first step towards cleaning mkimage code for kwbimage
support in clean way. Current mkimage code is very specific to
uimge generation whereas the same framework can be used to
generate other image types like kwbimage.
For this, the architecture of mkimage code need to modified.

Here is the brief plan for the same:-
a) Abstract image specific code to saperate file (this patch)
b) Move image header code from mkimage.c to respective
	image specific code
c) Implement callback function for image specific functions
d) Add kwbimage type support to this framework

In this patch-
1. Image specific code abstracted from mkimage.c to
	default_image.c/h to make mkimage code more generic
2. struct mkimage_params introduced to pass basic mkimage
	specific flags and variables to image specific functions

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>