spl: spl_legacy: fix the use of SPL_COPY_PAYLOAD_ONLY

If the payload is compressed, SPL_COPY_PAYLOAD_ONLY should always be set
since the payload will not be directly read to its load address. The
payload will first be read to a temporary buffer, and then be decompressed
to its load address, without image header.

If the payload is not compressed, and SPL_COPY_PAYLOAD_ONLY is set, image
header should be skipped on loading. Otherwise image header should also be
read to its load address.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>