lantiq: fix mtd_split for brnboot firmware images

lantiq: fix mtd_split for brnboot firmware images

The existing code only tries to split a parition with name "rootfs"
while actually looking for kernel+rootfs, where usually "firmware" is
used as a partition name. This doesn't make sense, as even if there
was a partition 'rootfs' given by mtdparts or via device-tree, this
code would lead to another partition called 'rootfs' being created
which would not be properly distinguished in the current user-space
code.

While potentially we could use CONFIG_MTD_SPLIT_FIRMWARE_NAME to make
that configurable, the lack of users of brnboot and the fact that we
can freely decide the partition name made me simply change the
hard-coded string constant from "rootfs" to "firmware".

(I'm going to add support for ARV7506PW which run brnboot and needs
this change)

SVN-Revision: 40324