command context: fix errors when running certain commands on startup

command context: fix errors when running certain commands on startup

Various commands, e.g. "arm mcr xxxx" would fail if invoked upon startup
since it there was no command context defined for the jim interpreter
in that case.

A Jim interpreter is now associated with a command context(telnet,
gdb server's) or the default global command context.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>