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)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40324 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/lantiq/patches-3.10/0100-mtd-split.patch

@@ -34,7 +34,7 @@
 +	size_t len;
 +	int ret;
 +
-+	if (strcmp(name, "rootfs") != 0)
++	if (strcmp(name, "firmware") != 0)
 +		return;
 +	while (part_size < end) {
 +		long size_min = part_size - 0x400 - 12 - offset;