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: NYork Sun <yorksun@freescale.com>
Signed-off-by: NBryan Wu <pengw@nvidia.com>