image: Rename SPL_SHAxxx_SUPPORT to SPL_FIT_SHAxxx

These option are named inconsistently with other SPL options, thus making
them incompatible with the CONFIG_IS_ENABLED() macro. Rename them.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>