blk: Make use of CONFIG_HAVE_BLOCK_DEVICE more

When we do not have CONFIG_BLK (or SPL/TPL) enabled there are very few
cases where we need the blk_legacy code linked in. To catch these, build
when we have CONFIG_HAVE_BLOCK_DEVICE set.  In addition, we only need
cmd/blk_common.o to be linked in when we have CONFIG_HAVE_BLOCK_DEVICE
set, so make use of that directly.

Signed-off-by: Tom Rini <trini@konsulko.com>