package/nodejs: Fixed npm global install

Set npm_config_prefix to $(TARGET_DIR)/usr so that npm -g will correctly
install global modules to $(TARGET_DIR)/usr/lib/node_modules.

By using npm -g to install global modules npm will now automatically create
the symlinks to executables under $(TARGET_DIR)/usr/bin.

See https://docs.npmjs.com/misc/config#global and
https://docs.npmjs.com/files/folders

 for information on how the prefix
setting is used by global installs.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>