treewide: use more descriptive names for concatenated partitions

treewide: use more descriptive names for concatenated partitions

A few devices in ath79 and ramips use mtd-concat to concatenate
individual partitions into a bigger "firmware" or "ubi" partition.

However, the original partitions are still present and visible,
and one can write to them directly although this might break the
actual virtual, concatenated partition.

As we cannot do much about the former, let's at least choose more
descriptive names than just "firmwareX" in order to indicate the
concatenation to the user. He might be less tempted into overwriting
a "fwconcat1" than a "firmware1", which might be perceived as an
alternate firmware for dual boot etc.

This applies the new naming consistently for all relevant devices,
i.e. fwconcatX for virtual "firmware" members and ubiconcatX for
"ubi" members.

While at it, use DT labels and label property consistently, and
also use consistent zero-based indexing.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>