bitbake: fetch: npm: Use temporary file for empty user config

Always use a temporary file for the user config 'NPM_CONFIG_USERCONFIG'
because npm otherwise failed if configs and npmrc aren't set:

double-loading config "/dev/null" as "global", previously loaded as "user"

(Bitbake rev: 9f272ad7f76c1559e745e9af686d0a529f917659)

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I24c35d1393495e1874573cd731f84c41382ee95e