mkh3cvfs: add filesystem tool for H3C devices

mkh3cvfs: add filesystem tool for H3C devices

The bootloader can only boot images from a custom filesystem which
normally spans most of the flash on the device. This tool creates a
filesystem image containing a single file (which should be a firmware
image created using mkh3cimg).

The size of the filesystem is hardcoded in the bootloader. However, an
image that is only slightly larger than its content is sufficient, as
long as no write operations are performed in the bootloader menu. This
allows to append further data (i.e. rootfs) after the end of the
filesystem image.

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