recipetool: create: refactor code for ensuring npm is available

recipetool: create: refactor code for ensuring npm is available

Across devtool and recipetool we had an ugly set of code for ensuring
that we can call an npm binary, and much of that ugliness was a result
of not being able to run build tasks when tinfoil was active - if
recipetool found that npm was required and we didn't know beforehand
(e.g. we're fetching from a plain git repository as opposed to an npm://
URL where it's obvious) then it had to exit and return a special result
code, so that devtool knew it needed to build nodejs-native and then
call recipetool again. Now that we are using real build tasks to fetch
and unpack, we can drop most of this and move the code to the one place
where it's still needed (i.e. create_npm where we potentially have to
deal with node.js code in a plain source repository).

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