bloblist: Make BLOBLIST_ALLOC the default

bloblist: Make BLOBLIST_ALLOC the default

We want to encourage people to use an allocated bloblist since it is
more flexible than a fixed one. Make this the default, being sure not to
change existing users.

The unit tests require BLOBLIST_FIXED so add a dependency in the
Makefile to avoid build errors.

All sandbox builds require BLOBLIST_FIXED so make that the default for
sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>