Adding support for custom SEL records

[Lei Yu]

This mail is to propose some changes in openbmc to support custom SEL records.

* Background
The phosphor-sel-logger is the service to provide methods (IpmiSelAdd,
IpmiSelAddOem) to add custom SEL records.
However:
1. It only sends the log into the journal;
2. It depends on
meta-intel/meta-common/recipes-extended/rsyslog/rsyslog/rsyslog.conf
to filter the journal log and write to "/var/log/ipmi_sel"
3. It depends on intel-ipmi-oem to parse the "/var/log/ipmi_sel" to
provide the SEL entries.

In short, phosphor-sel-logger only works with the meta-intel layer and
intel-ipmi-oem.

To make it support general usage, several changes are submitted to gerrit:
https://gerrit.openbmc-project.xyz/q/topic:%22dev-add-custom-sel%22+(status:open%20OR%20status:merged)

Some details:
1. It adds SEL.errors.yaml in PDI.
2. With SEL_LOGGER_SEND_TO_LOGGING_SERVICE option, phosphor-sel-logger
creates entries in the logging service without callout path, instead
of sending to the journal.
3.  In phosphor-host-ipmid, it adds extra logic to parse the logging
entries and convert to SEL entries.

Comments are welcome.

-- 
BRs,
Lei YU