sys/console: Decouple line buffers from console history entries
sys/console: Decouple line buffers from console history entries

Console history will just keep pointers to "external" buffers. This will
make easier to move history elements.

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