archiver: avoid using machine variable as it breaks multiconfig

STAGING_KERNEL_DIR uses the MACHINE name so it breaks the multiconfig
and in this cases it will run the shared recipes twice, one for each
machine.

STAGING_KERNEL_DIR it's been introduced in commit 5487dee2

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>