[OE-core] backports for jethro

[Tim Kilbourn]

What is the policy for backporting changes to released branches like
jethro? I'm hitting an issue that was fixed in 2015 on master, but
didn't get included in jethro 2.0.1. Is there any chance of having
this cherrypicked so I can get the fix from the jethro branch in git?

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commit 9abcd309c098558360cde2bff65be840ead25f83
Author: Robert Yang <liezhi.yang at windriver.com>
Date:   Tue Nov 24 05:20:38 2015 -0800

    boot-directdisk.bbclass: remove HDDIMG before create

    Fixed when rebuild:
    mkdosfs: file /path/to/hdd.image already exists

    Signed-off-by: Robert Yang <liezhi.yang at windriver.com>
    Signed-off-by: Ross Burton <ross.burton at intel.com>