fs: change the way the device table is configured

Until now, the location of the device table was specified by a
variable in board Makefiles. Unfortunately, this variable is not
accessible from fs/common.mk, since the target/ code is included
*after* fs/common.mk.

Anyway, the general idea is to move away from these boards Makefile,
and provide configuration option for things like the device table
location.

Therefore, this patch adds a BR2_ROOTFS_DEVICE_TABLE option which
allows to specify which device table should be used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>