nodejs: fix build when python3 is selected in Buildroot

nodejs: fix build when python3 is selected in Buildroot

When Python 3 is selected in Buildroot, the host/usr/bin/python
symlink of the host Python interpreter points to python3. Packages
that need to use the host Python 2 interpreter have to use python2.

In commit 40218a16526946f35dab0d05f379f71a01f29530 ("nodejs: force
python interpreter"), Samuel made some changes to the nodejs package
to use python2. One part of the changes is to sed a .gyp file to
replace the string 'python' by the path to python2. However, this
operation is done *after* calling the configure script, so it has in
fact no effect. Putting this sed before calling the configure script
fixes the problem.

However, there is a better solution: the nodejs build system has a
mechanism of variables, and it already defines a python variable
according to the environment variable PYTHON being passed. So this
patch instead adds a new patch to nodejs to use this python variable.

Fixes:

  http://autobuild.buildroot.org/results/aff/affd7300895ec400de50a33d51b4e94e15d63341/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>