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>
diff --git a/common/Kconfig b/common/Kconfig
index 0e8c44f..7685914 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1047,6 +1047,8 @@
 
 choice
 	prompt "Bloblist location"
+	default BLOBLIST_FIXED if SANDBOX
+	default BLOBLIST_ALLOC
 	help
 	  Select the location of the bloblist, via various means.