Enhancement: Pass alternative style.json name to build-style

[citizenfish]

At present make build-style can only work from the style.json file in each layer subdirectory.

This makes it difficult to maintain multiple styles , especially when maputnik is used to edit.

If get_ts_lyr_style_json_fp could be modified to accept an alternative to style.json then multiple styles could be maintained in the layer directory

In addition it would be great if the split command in style-tools could be added to openmaptiles Makefile to support a workflow where the style is built, edited in maputnik and then written back to the layer directories