MYNEWT-787 serial boot loader - nmp buf overruns
MYNEWT-787 serial boot loader - nmp buf overruns

The serial boot loader fails to process some newtmgr commands:

1. The image list response is too large for the outgoing buffer.
2. The "new" newtmgr sends large image chunks in its image upload
   requests. These chunks are too large for the serial boot loader's
   input buffer.

The fixes are as follows:
1. Increase the outgoing buffer size from 48 to 80.
2. Increase the incoming buffer size from 128 to 512.
3. Return an error rather than overrun the output buffer.

andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,IMGJulian/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core