Dump manager: Add system_dump_policy object

This commit adds the system_dump_policy object to the
xyz.openbmc_project.Settings. This is used to enable or disable
the system dump. Default value is true

Tested by:
busctl tree xyz.openbmc_project.Settings
└─/xyz
  └─/xyz/openbmc_project
    ├─/xyz/openbmc_project/dump
    │ └─/xyz/openbmc_project/dump/system_dump_policy

busctl introspect xyz.openbmc_project.Settings /xyz/openbmc_project/dump/system_dump_policy

(From meta-ibm rev: ad88bbcbbd01d92773f61b1712f5e458f1b661bd)

Signed-off-by: Sunitha Harish <sunharis@in.ibm.com>
Change-Id: Ieae8a01a266ba8ff2d984b1a1b2ea9ada243d439
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>