bootm: Use selected configuration for ramdisk and fdt

If a specific configuraion is selected by the bootm command, e.g. with
'bootm 84000000#recoveryconf' we must honour this for not just the kernel,
but also the ramdisk and FDT.

In the conversion to using a common fit_image_load() function for loading
images from FITs (commits a51ec63b and 53f375fa) this feature was lost.
Reinstate it by passing the selected configuration back from
fit_image_load() to boot_get_kernel(), then use this configuration
(which is stored in images->fit_uname_cfg) in both boot_get_ramdisk()
and boot_get_fdt().

Signed-off-by: Simon Glass <sjg@chromium.org>