Adds support to prepare rescue ramdisk to iRMC PXE boot interface,
which is a subclass of PXE boot interface by adding the new argument.
Also adds unit tests to confirm iRMC PXE boot interface passes the
same tests as generic PXE boot interface.
Change-Id: I1b4aeb207772a09cacf0bff910be4061ec5c74ee
Related-Bug: #1526449