[new uImage] Add dual format uImage support framework

This patch adds framework for dual format images. Format detection is added
and the bootm controll flow is updated to include cases for new FIT format
uImages.

When the legacy (image_header based) format is detected appropriate
legacy specific handling is invoked. For the new (FIT based) format uImages
dual boot framework has a minial support, that will only print out a
corresponding debug messages. Implementation of the FIT specific handling will
be added in following patches.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>