obmc-init: Add factory reset support

In obmc-init.sh, there is "clean-rwfs-filesystem" that clean the rwfs
during boot, but it backs up the files in whitelist, so it is not a
*real* factory reset.

Add a new "factory-reset" option, and when it exists, erase the rwfs
without restoring files in whitelist.

Tested: Set openbmconce=factory-reset in u-boot env, and verify the rwfs
        is erased during boot without restore;
        Set openbmconce=clean-rwfs-filesystem in u-boot env, and verify
        that rwfs is erased with whitelist restored.

Change-Id: Ifdc76da46fe273e9c942f30444173e51f8106fcc
Signed-off-by: Lei YU <mine260309@gmail.com>