ktest: Add special variable ${KERNEL_VERSION}

Add a special variable that can be used in other variables called
${KERNEL_VERSION}. This will embed the current kernel version into
the variable. For example:

WARNINGS_FILE = ${OUTPUT_DIR}/warnings-${KERNEL_VERSION}

If the current version is v3.8 then the WARNINGS_FILE will become

  ${OUTPUT_DIR}/warnings-v3.8

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