MYNEWT-858 Collect all package lflags during link.
MYNEWT-858 Collect all package lflags during link.

If you add the following to a test package:

```
pkg.lflags:
    - "-foo"
```

and then test the package with `newt test`, the flag does not get passed
to gcc during linking.

Newt currently only reads lflags from the following package types:
* target
* app
* bsp
* compiler

Now, all packages' lflags are collected and specified during linking.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt