ktest: Add POST_INSTALL to allow initrds to be created

Add a POST_INSTALL option that runs after the build and install
but before rebooting to the test kernel. This alls the user to
run a script that will install an initrd (or anything else that may
be special) before booting.

An environment variable KERNEL_VERSION is set.

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