recipetool: create: allow license variable handling to be rerun

If you make adjustments to the source tree (as create_npm.py will be)
then you will need to re-run the license variable handling code at the
end so that we get all of the files that should go into
LIC_FILES_CHKSUM if nothing else. Split out the license variable
handling to a separate function in order to allow this.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>