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: NSteven Rostedt <rostedt@goodmis.org>

Added sample.conf as a nice document to show new users.

Use a %default hash to separate out the options that are default
and allow us to complain about options being set twice.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>