No way to choose SRK source index

[ofacklam]

If I would like to use SRK2 or 3 or 4 to authenticate the imx-boot image, my understanding is that I would have to change the `Source index" variable in this block of the CSF.

[Install SRK]
    # Index of the key location in the SRK table to be installed
    File = "CST_SRK"
    Source index = 0

Would it be possible to provide a way to choose this through a variable, the same way the CSFK and IMG key files can be chosen?