pimgen: Split argument parsing logic
pimgen: Split argument parsing logic

Move the non-argument parsing logic out of the main function,
in preparation for enabling additional command options.

Change-Id: Ib2cd75e432e11982a85a824e607ddf2568de63a9
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-inventory-manager,openbmc/phosphor-inventory-manager