powerpc/nvram: Improve partition removal

Existing code is nasty, has bugs etc... rewrite the function
more simply, and make it take the signature and optional
name of the partitions to remove as arguments, thus making
it a more generic utility.

We also try to remove a log partition that we find and is too
small rather than creating a duplicate.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>