recipetool: allow plugins to set LICENSE and LIC_FILES_CHKSUM

recipetool: allow plugins to set LICENSE and LIC_FILES_CHKSUM

We were being a bit prescriptive in setting LICENSE and
LIC_FILES_CHKSUM. We can't always trust what's in the metadata
accompanying some source which plugins will almost always be pulling
from, however we do want to allow plugins to set the LICENSE and
LIC_FILES_CHKSUM values. Merge what we find in our license file scan
with what the plugin sends back.

Additionally, plugins can now add a "license" item to the handled list
in order to inhibit the normal LICENSE / LIC_FILES_CHKSUM handling if
they have already taken care of it completely.

Thanks to Mark Horn <mark.d.horn@intel.com> for prompting, testing and
fixing this patch.

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