kernel: enable namespaces

We've been already trying to enable NET_NS (and others), but all
namespaces are apparently guarded by this one as well.

As a bonus, this should fix that crash triggered by systemd-networkd
v246 with SYSTEMD_LOG_LEVEL=debug .

Reported on the linux-fsdevel ML ("Null pointer dereference in nsfs_evict with
CONFIG_NET_NS=n triggered via systemd-networkd's debugging"), and thanks
a ton to Vlastimil Babka <vbabka@suse.cz> for helping me debug this.

Change-Id: Ib6097d009ce2b503f4c53df6eb100b0018b4611f
Bug: https://lore.kernel.org/linux-fsdevel/aa151d26-37a1-4836-b28f-33c46d9894e2@cesnet.cz/T/#u
diff --git a/board/czechlight/common/linux.fragment b/board/czechlight/common/linux.fragment
index 2948be1..ac086e1 100644
--- a/board/czechlight/common/linux.fragment
+++ b/board/czechlight/common/linux.fragment
@@ -28,6 +28,7 @@
 CONFIG_FW_LOADER_USER_HELPER=n
 CONFIG_DMIID=y
 CONFIG_BLK_DEV_BSG=y
+CONFIG_NAMESPACES=y
 CONFIG_NET_NS=y
 CONFIG_DEVPTS_MULTIPLE_INSTANCES=y
 CONFIG_USER_NS=y