classes/npm: use npm install to do installation

classes/npm: use npm install to do installation

Using "npm install" instead of "cp -a" is the more correct thing to be
doing here, and ensures that symlinks for executable scripts are put
into ${prefix}/bin. (I'd prefer ${bindir}, but npm does not allow
specifying paths at that level - only a prefix.)

Fixes [YOCTO #10460].

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