Factor out deploy info from PXE driver

This factors the code to get the deploy kernel and ramdisk info out
of the PXE driver, as this can be shared between the PXE and agent
drivers.

Change-Id: I2c3722d091f02259aa9e6cabebaabf25eead8bc2
(cherry picked from commit 8368bf9c66eea3a5f4706c867e266e00a5886213)
Merge "Factor out deploy info from PXE driver"