rauc: Clean persistent TMPDIR before service start

[t-mon]

TMPDIR points to /srv/rauc-tmp, so it persists across reboots.
Clear the directory in ExecStartPre to avoid stale RAUC downloads filling /srv.

What a nice oxymoron: "persistent TMPDIR"

[mhei]

Nitpick: we could "save" the shell glob stuff when we just do

ExecStartPre=/bin/rm -rf "$TMPDIR"
ExecStartPre=/bin/mkdir -p "$TMPDIR"

Or did I miss anything?
Other than this, LGTM

[t-mon]

If the TMPDIR does not exist for whatever reason, the RAUC service would failed to start because rm would return non zero value. The shell glob stuff is only required for using the wildcard rm of the folder content, i thought this way it is the most fail-save, independent of the folder existence or it's content.

[mhei]

rm -rf ... always returns success in my tests, even for non-existent dirs?
I was only concerned regarding the (here very very unlikely) case that TMPDIR does not make it into the call and is empty because then /bin/sh -c 'rm -rf "$TMPDIR"/*' would become effective /bin/sh -c 'rm -rf /*' 😄
Feel free to merge as-is.