spl: LOAD_FIT_FULL: Support 'kernel' and 'firmware' properties

The 'firmware' property of a config node takes precedence over the
'kernel' property. 'standalone' is deprecated. However, give users a
couple of releases where 'standalone' still works, but warns loudly.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>