image: Drop if/elseif hash selection in calculate_hash()

calculate_hash() would try to select the appropriate hashing function
by a if/elseif contruct. But that is exactly why hash_lookup_algo()
exists, so use it instead.

This does mean that we now have to 'select HASH' to make sure we get
the hash_lookup_algo() symbol. However, the change makes sense because
even basic FITs will have to deal with "hash" nodes.

My only concern is that the 'select SPL_HASH' might cause some
platform to grow above its SPL size allowance

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
[trini: Make FSL_CAAM be implied only on ARM && SPL]
Signed-off-by: Tom Rini <trini@konsulko.com>