stx-extensions: Fix the systemd unitdir and the arguments for kernel.core_pattern

stx-extensions: Fix the systemd unitdir and the arguments for kernel.core_pattern

Fix the systemd unitdir and the arguments for kernel.core_pattern,
or it fails with:

- [14487.239126] Core dump to |/usr/lib/systemd/systemd-coredump 1038359 0
  0 11 1593333694 mtcAgent pipe failed

- [ 2573.933792] systemd-coredump[250065]: Not enough arguments passed by
  the kernel (6, expected 7).

Issue-ID: INF-180
Issue-ID: INF-181
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: I420c8c669a36596de64709f136f77be434583008