Update html build script subprocess.call

[bmanifold]

First off, thank you for creating this framework, it's been a ton of fun to use.

I, and a few others working on a small project together, came across this bug while trying to create some custom fields in the configuration manager page. We were running in to a bug where the npm commands in the preBuildHTML.py script weren't getting run as intended.

We've been using PlatformIO to compile (on both Mac and Linux).
PlatformIO: Core-5.2.3

The versions of python we've been using:
Mac - python:3.8.2
Linux - python:3.7.3

Below is a copy of the commit message to give more detail on the changes. Please let me know if any more information is needed/wanted and thank you for considering this PR.

Why:

• While trying to rebuild the HTML for custom attributes, the
  preBuildHTML.py script was failing when calls were made to
  subprocess.call. Under the hood the subprocess.call function
  calls Popen. The following is from the documentation page of the
  subprocess library:

  https://docs.python.org/3/library/subprocess.html#subprocess.Popen

  If shell is True, it is recommended to pass args as a string rather than as a sequence.
  

  While the documentation says that passing a sequence of program
  arguments should work as intended, building on linux and Mac both
  failed when using the sequence and succeeded when using a single
  string.

This change addresses the need by:

• Updating all calls to subprocess.call in preBuildHTML.py to use
  single string instead of list as the first argument.

[maakbaas]

Thanks for the detailed PR, I've merged it into the repository