deploy.bbclass: Clean DEPLOYDIR before do_deploy

do_deploy should clean up ${DEPLOYDIR} before running, just like do_install
cleans up ${D} before running. This reduces the risk of DEPLOYDIR being
accidentally contaminated by files from previous runs, possibly even with
different config, in case of incremental builds.

It is convenient to have this in deploy.bbclass, so it doesn't have to be
duplicated in every recipe, considering for example meta-freescale, which
has 23 affected recipes.

All recipes using deploy.bbclass (grep -r 'inherit .*deploy') in poky,
meta-openembedded and meta-freescale look like they either benefit from
this or are at least not affected negatively by it. The only exception
I've noticed was uboot-sign.bbclass, which was however fixed by the
previous patch.

Signed-off-by: Daniel Klauer <daniel.klauer@gin.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>