dreport: Populating missing fields of header

Changes:
-Added time.
-Added bmc serial number
-Added dump entry

Result:
Tested the dump header using busctl command and dreport script.
Able to add dump header on top of the <obmcdump>.tar.xz file.

In hexdump o/p can see the time, serial number and entry details.

Separated out the header from the output file using dd command:
dd bs=628 skip=1 if=<obmc dump>.tar.xz of=openbmc.tar.xz

The new output file doesn't have the header now and can be unzipped.
Post unzipping the files in the dump can be seen.

Signed-off-by: Chirag Sharma <chirshar@in.ibm.com>
Change-Id: Ia8af50cebb09350d93a631531495081c24667f26