kernel: update and enable full 1GB of RAM on Clearfog Base

I had no idea that I need CONFIG_HIGHMEM to be able to even *see* more
than about 750 MB of RAM on this particular ARM CPU, but hey, here it
goes. (At least there was an info message in dmesg.)

Before:

 # free
               total        used        free      shared  buff/cache   available
 Mem:         769276       47968      407976      269304      313332      438664
 Swap:             0           0           0

Now:

 # free
               total        used        free      shared  buff/cache   available
 Mem:        1029352       49392      894400       43168       85560      913896
 Swap:             0           0           0

Also update to the latest stable (our patches are available as cesnet/2020-04-29).

Change-Id: I6cabd684fcfbe78a2718130ae46ca7f77b4c8228
diff --git a/board/czechlight/clearfog/linux.fragment b/board/czechlight/clearfog/linux.fragment
index 744e96d..71cd8bd 100644
--- a/board/czechlight/clearfog/linux.fragment
+++ b/board/czechlight/clearfog/linux.fragment
@@ -1,3 +1,6 @@
+# enable 1GB of RAM, not just 750 MB
+CONFIG_HIGHMEM=y
+
 # this is a part of the Clearfog Base carrier
 CONFIG_SENSORS_MCP3021=y