Removed support for "v11_ddr2_512mb" bootloader variant

Units with this configuration were not released to customers.
Use "system partition" as the name for the partition holding the kernel

This is to be consistent with our other tooling. At some point,
we will be putting the rootfs on the system partition as well.
Flush cache before verifying installed file integrity

Otherwise we'd be verifying the contents of the cache, most likely,
as the files we install will all fit in memory.
Phrased "Verifying" progress message differently
Force sync after kernel rename

While the partition will be unmounted immediately after, it is possible
the same partition is mounted as /media/system as well, in which case
I'm not sure whether the unmount will force a sync.