OpenPOWER: Do not delete dump entries during mp reboot

The dump entries for the dumps stored in the host memory
should not be deleted during a memory preserved reboot.

There is a file created during memory preserved reboot
this service will not be active if the file is present

Additional change to make sure the file which indicates
memory preserving ipl is already created
https://gerrit.openbmc-project.xyz/c/openbmc/openpower-proc-control/+/51515



Tests:
During memory preserving reboot:
Feb 24 11:38:53 systemd[1]: Finished Start memory preserving reboot host0.
Feb 24 11:38:53 systemd[1]: Condition check resulted in Remove host dump entries during poweroff being skipped.

In normal poweroff
Feb 24 11:52:08 ever6bmc systemd[1]: Reached target Stop Host0 (Pre).
Feb 24 11:52:08 ever6bmc systemd[1]: Starting Remove host dump entries during poweroff...
Feb 24 11:52:08 ever6bmc systemd[1]: Starting Stop instructions for host0...
Feb 24 11:52:08 ever6bmc phosphor-dump-manager[496]: System dump delete id(3) srcdumpid(2)
Feb 24 11:52:08 ever6bmc phosphor-dump-manager[496]: System dump entry with id(3) is deleted
Feb 24 11:52:08 ever6bmc systemd[1]: Finished Remove host dump entries during poweroff.

Signed-off-by: Dhruvaraj Subhashchandran <dhruvaraj@in.ibm.com>
Change-Id: Ic4269ee3322c4b8b77ea072a97e325895feed20f