linux: Unify dmesg size

On clearfog, `dmesg` won't output everything even when entered within a
minute of the boot because its default value of 14 is way too low. Given
that x86 uses 18 and that mvebu_v5 is even at 19, let's allow for some
headroom here -- we have 1GB of RAM, anyway.

Change-Id: I467b047406cab96bb1af4cf5c37b3782f67e1935
diff --git a/board/czechlight/common/kconfig b/board/czechlight/common/kconfig
index 87a0c85..f63a068 100644
--- a/board/czechlight/common/kconfig
+++ b/board/czechlight/common/kconfig
@@ -62,3 +62,6 @@
 
 # protecting /etc
 CONFIG_OVERLAY_FS=y
+
+# better logging
+CONFIG_LOG_BUF_SHIFT=18