apps/boot: Add `sys/log/stub` dependency
apps/boot: Add `sys/log/stub` dependency

A recent commit (`b10cbea5ef882e7f91d1c34ffcf2506d3e183003`) imposes the
`LOG` API requirement on the `sys/mfg` package. To fix broken builds,
make the boot app depend on `sys/log/stub`.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core