fs_mgr: allow boot_devices to be supplies on the kernel commandline

boot_devices in device tree is our last remaining device tree entry
for specifying the kernel commandline.  Since this doesn't make sense
to be included in the fstab, especially as there is no way to
differentiate between different boot devices within a single device
tree as some devices do, it is moved to the kernel commandline.

Bug: 117933812
Test: boot blueline without boot_devices in DT
Change-Id: If498836ae2ef14e68ff761338ef352c20c4a7b3d