Set default partition filesystem to null and add unit tests for it

[mschwan-phytec]

Fix function declaration style for check_partition_alignment().

The default filesystem for partitions is now null, instead of fat32. This is now in line with the documentation, which has been using null, too.

Add a new test for checking all supported partition filesystems, including null and the default value, which should now also be null.

[landerweit-phytec]

The documentation does not mention any default for the filesystem. null is not mentioned at all. https://partup.readthedocs.io/en/latest/layout-config-reference.html#partitions

[landerweit-phytec]

Code and test look good.

[mschwan-phytec]

The documentation does not mention any default for the filesystem. null is not mentioned at all. https://partup.readthedocs.io/en/latest/layout-config-reference.html#partitions

Right. I could swear I read it in that section as well.

What I had in mind was probably this: https://partup.readthedocs.io/en/latest/layout-config-reference.html#supported-file-types See the ext[234] description:

This overrides any existing filesystem, so it should be specified as filesystem: null or not be specified at all.

This implies the default would be null. You are right, the documentation doesn't explicitly mention this. I'll make this more clear in an additional commit.