Drop config.h include in tools/imximage.h

"make tools-all" should allow building tools such as mkimage and the new
imximage without any config, but imximage.c currently fails to build
with:
imximage.h:27:20: error: config.h: No such file or directory

config.h is not needed in imximage.h nor in imximage.c, and imximage.h
is only included from imximage.c, so drop this include to fix the build.

Signed-off-by: Loc Minier <loic.minier@linaro.org>