fs/common.mk: Strip extra whitespace when device table is empty

Since ROOTFS_DEVICE_TABLES is a concatenation of two strings separated by
spaces, there will always be a space in it, which means it's never empty.
Therefore, when testing for empty, the condition never evaluate to false.

The following change fixes this problem; it runs qstrip on the overall
combination of the variables, causing the space to be removed if it's the
only thing left.

Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Thomas De Schampheleire <patrickdepinguin+buildroot@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>