src/context: remove surrounding whitespace from system variant
src/context: remove surrounding whitespace from system variant

This was always intended to be a short string without whitespace. Remove
the whitespace from when preparing the context.

In the manifest, the variant is part of the image section name, so
intentional surrounding whitespace would be very unusual.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc