kernel: mtdsplit: add support for H3C VFS filesystem

kernel: mtdsplit: add support for H3C VFS filesystem

The bootloader on some H3C devices (for example HPE 1920 switches) only
supports booting from flash by reading an image from an "VFS" filesystem
which spans most of the available flash. The filesystem size is hard-
coded in the bootloader. However, as long as no write operations are
performed in the bootloader menu, it is sufficient if the start of the
partition contains a valid filesystem with the kernel image.

This mtdsplit parser reads the size and location of the kernel image and
finds the location of the rootfs stored after it. It assumes that the
filesystem image matches the layout of one generated by mkh3cvfs, with
a filename of "openwrt-kernel.bin" for the kernel image.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>