ktest: Implement our own force min config

Using the build KCONFIG_ALLCONFIG environment variable to force
the min config may not always work properly. Since ktest is
written in perl, it is trivial to read and replace the current
config with the configs specified by the min config.

Now the min config (and add configs) are read by perl and before
a make is done, these configs in the .config file are replaced
by the version in the min config.

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