Save PKI token in a file for PXE deploy ramdisk

Vendor passthru API should be used for PXE deploy from ramdisk,
but we must have a valid PKI token for authorization. PKI token
saved in a file in the tftp root for further read from the ramdisk.

Change-Id: I8e5f1cf0a0c48874b694f24226613a8db4d49cd1
Merge "Save PKI token in a file for PXE deploy ramdisk"