tools: mkimage: split code into core, default and FIT image specific

This is a first step towards reorganizing the mkimage code to make it
easier to add support for additional images types. Current mkimage
code is specific to generating uImage and FIT image files, but the
same framework can be used to generate other image types like
Kirkwood boot images (kwbimage-TBD). For this, the mkimage code gets
reworked:

Here is the brief plan for the same:-
a) Split mkimage code into core and image specific support
b) Implement callback functions for image specific code
c) Move image type specific code to respective C files
       Currently there are two types of file generation/list
       supported (i.e uImage, FIT), the code is abstracted from
       mkimage.c/.h and put in default_image.c and fit_image.c;
       all code in these file is static except init function call
d) mkimage_register API is added to add new image type support
All above is addressed in this patch
e) Add kwbimage type support to this new framework (TBD)
This will be implemented in a following commit.

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Edit commit message, fix coding style and typos.
Signed-off-by: Wolfgang Denk <wd@denx.de>