Supporting new interfaces in phosphor-ipmi-flash

[Troy Lee]

Hi team,

For security consideration, user might want to disable AST2500/AST2600 P2A functionality by default. To compensate the effect to phosphor-ipmi-flash, we're planning to support two alternative in-band firmware upgrade over PCIe for AST2500/AST2600 (AST2520 and AST2620 are excluded):
 - Through a reserved **VGA** memory on BAR[0], or
 - Through a reserved **PCIe** shared memory on BAR[1]

The usage pretty much the same as P2A, but it runs on different BAR, offset and length.
This will involves modifying phosphor-ipmi-flash/[tools|bmc]. Should I create new **interfaces**, e.g. astpcie/astvga?

Thanks,
Troy Lee