kernel: add parser finding rootfs after CFE bootfs

kernel: add parser finding rootfs after CFE bootfs

It's required for BCM4908. It cannot use "bcm-wfi-fw" parser because
that one requires *two* JFFS2 partitions which is untested / unsupported
on the BCM4908 architecture. With a single JFFS2 partition "bcm-wfi-fw"
parser will:
1. Fail to find "vmlinux.lz" as it doesn't follow "1-openwrt" file
2. Create partitions that don't precisely match bootfs layout

The new parser is described in details in the MTD_SPLIT_CFE_BOOTFS
symbol help message.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>