wic:direct.py: ignore invalid mountpoints during fstab update

wic fstab-update creates invalid entries for partitons that are not supposed to
be mounted from userspace eg u-boot partitions.

The following lines were added to fstab on a rock-pi-4:
/dev/mmcblk1p1 loader1 vfat defaults 0 0
/dev/mmcblk1p2 reserved1 vfat defaults 0 0
/dev/mmcblk1p3 reserved2 vfat defaults 0 0
/dev/mmcblk1p4 loader2 vfat defaults 0 0
/dev/mmcblk1p5 atf vfat defaults 0 0
/dev/mmcblk1p6 /boot vfat defaults 0 0

With this patch only valid entries should be added
/dev/mmcblk1p6 /boot vfat defaults 0 0

Signed-off-by: MarkusVolk <f_l_k@t-online.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>