package/nodejs: Define NPM command for other packages to use

Other nodejs-related packages will need to call npm with the same set of
arguments as is currently used by the nodejs package itself.

To avoid duplicating this code, set the NPM variable so those packages can
re-use it.

Signed-off-by: Martin Bark <martin@barkynet.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>