Makefile: Add 'make shell' target
Makefile: Add 'make shell' target

The 'make shell' target loads fernly onto the board, then opens an
interactive shell with it.

Signed-off-by: Sean Cross <dd28e116ed3e781f3ae781acae9fb18383f35c2f@kosagi.com>

mandl/fernly,xobs/fernly,mandl/fernly,pfalcon/fernly,xobs/fernly,urjaman/fernly,pfalcon/fernly,mandl/fernly,urjaman/fernly,urjaman/fernly