InstallUpstreamKernel.py: fix initrd file string
InstallUpstreamKernel.py: fix initrd file string

Parse the initrd file name correctly for kexec to use the proper
initrd file, in a case where we have 2 initrd file with same kernel
release version, the existing script choses the last file, this code
will make sure it uses the exact initrd file for kexec

Signed-off-by: Abdul Haleem <d72979407b2438b90c42979d11f88d04d190c5d3@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework