fs/common.mk: make sure that static devices from packages are created

fs/common.mk: make sure that static devices from packages are created

The static devices defined by packages are currently added to the full
device table when two conditions are met:

 (1) ROOTFS_DEVICE_TABLES is non-empty
 (2) BR2_ROOTFS_DEVICE_CREATION_STATIC=y

(2) is obviously correct. However, depending on (1) is not correct: if
the user doesn't provide any custom permission table and custom device
table, then ROOTFS_DEVICE_TABLES will be empty.

So instead, move the addition of the package-defined static devices
outside of condition (1), and have it only under condition (2).

Reported-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>