Commit b1f10b18eaf6c8009e86863ca4a26f429de97082 added the NPM_INSTALL_DEV
variable. But due to the use of simple assignment, a recipe can't override its
value.
(From OE-Core rev: b53d5094d5e05f9c1955c8565d777ad74f668f7e)
Signed-off-by: Jef Driesen <jef.driesen@niko.eu>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>