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/configs/czechlight_beaglebone_defconfig b/configs/czechlight_beaglebone_defconfig
index d43a1ab..8b859d3 100644
--- a/configs/czechlight_beaglebone_defconfig
+++ b/configs/czechlight_beaglebone_defconfig
@@ -22,7 +22,7 @@
 BR2_ROOTFS_POST_SCRIPT_ARGS="-c $(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/beaglebone/genimage.cfg"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.5.8"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.6.7"
 BR2_LINUX_KERNEL_DEFCONFIG="omap2plus"
 BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="$(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/common/linux.fragment $(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/beaglebone/linux.fragment"
 BR2_LINUX_KERNEL_DTS_SUPPORT=y