image: fix bootm failure for FIT image

Commit b3dd64f5 "bootm: use genimg_get_kernel_addr()" introduced
a bug for booting FIT image. It's because calling fit_parse_config()
twice will give us wrong value in img_addr.

Add a new function genimg_get_kernel_addr_fit() whichl will always
return fit_uname_config and fit_uname_kernel for CONFIG_FIT.
genimg_get_kernel_addr() will ignore those to parameters.

Reported-by: York Sun <yorksun@freescale.com>
Signed-off-by: Bryan Wu <pengw@nvidia.com>