add eraseblock header support for mkfs.jffs2

Hi,

This patch added the eraseblock header support for mkfs.jffs2.
In particular, the option "--eraseblock-header" is used to specify the eraseblock header size, the default is 28 bytes; "--no-eraseblock-headers" is used to specify that no EBH is written to each erase block.

The patch is at:
http://www.infradead.org/~forrest/mkfs.jffs2.patch

Thanks,
Forrest