classes: image_types: quote variable assignment needed by dash

The change in commit 39fc5030
("classes: image_types: apply EXTRA_IMAGECMD:squashfs* in oe_mksquashfs()")
assigns $@ to a local variable without quoting it. While this works with
bash, it fails with dash. Here, only the first token of $@ is assigned
to the variable, and the reamining tokens are passed as arguments to the
"local" keyword.

Fix it by adding the missing quotes.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 14ca134f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>