elf2bin.py objcopy cleanup

[mcspr]

Per https://docs.python.org/3/library/tempfile.html#tempfile.mkstemp

Unlike TemporaryFile(), the user of mkstemp() is responsible for deleting the temporary file when done with it.

read_segment()'s dumpfile was never deleted, producing 5 'segment' files each build.

Instead, this patch creates a temporary directory which is later used for objcopy output. TemporaryDirectory() context manager will handle the deletion.
Plus, some minor refactoring to reduce line length and add some pythonisms (hopefully, not too much)

Noticed this issue accidentally by inspecting a specific Windows PC install using WSL circa January. /tmp was ~2.5GB and there was a lot of files.

[earlephilhower]

Good fix, but needs to have #7348 merged before it can go in. The TemporaryDirectory method isn't available in Python2 (which is the only one installed on Macs until the very latest release AFAIK).

Using a mkdtemp call and cleaning up afterward would allow it to go in even if #7348 isn't merged as that's available since Python2.