init: adjustments for retrofitted dynamic partitions

The bootloader is still configured for system-as-root and passes cmdline
options that are incompatible with the retrofitted dynamic partitions.
Adjust the kernel to ignore those parameters to make the new setup work.

Issue: FP3-A11#146
Change-Id: Idb918a4936af898e267daab8cc7a80f94bce6fb8