tools/mkimage: fix compiler warnings, use "const"

This fixes some compiler warnings:
tools/default_image.c:141: warning: initialization from incompatible pointer type
tools/fit_image.c:202: warning: initialization from incompatible pointer type
and changes to code to use "const" attributes in a few places where
it's appropriate.

Signed-off-by: Wolfgang Denk <wd@denx.de>