ktest: Add options SWITCH_TO_GOOD and SWITCH_TO_TEST

For machines that do no use grub, it may be needed to update an
external image (tftp) before doing a reboot into either the
test image or the known good image.

The option SWITCH_TO_GOOD is added, where if it is defined, the
command that is specified as its value will be executed before
doing a reboot into a known good image.

The option SWITCH_TO_TEST is added, where if it is defined, the
command that is specified as its value will be executed before
doing a reboot into the test image.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>