swupdate_class: do not overwrite sw-description from WORKDIR
swupdate_class: do not overwrite sw-description from WORKDIR

If SRC_URI contains a file called sw-description, it overwrites the
one from WORKDIR.  Fix this.

Signed-off-by: Mans Rullgard <2c609ffb4e3675cc825ffc138f42668aebf8267d@mansr.com>

sbabic/meta-swupdate,sbabic/meta-swupdate