gdb: Only connect to remote target once when first sourced

gdb: Only connect to remote target once when first sourced

The gdb_grub script was originally meant to be run once when GDB first
starts up via the -x argument. So it runs commands unconditionally
assuming that the script has not been run before. Its nice to be able
to source the script again when developing the script to modify/add
commands. So only run the commands not defined in user-defined commands,
if a variable $runonce has already been set and when those commands have
been run to set $runonce.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>