clearfog: Introduce a /cfg partition for persistent changes

The idea is to store whatever important bits that need real persistence
in there.

Note that I am deliberately *not* using this as the upper directory for
the /etc overlay. I inherited that company-wide worrying about flash
lifetime, and hence I would like to limit the amount of writes to some
reasonable minimum. That doesn't play terribly well with SW such as
sysrepo which happily stores both persistent and run-time data on disk,
in the same directory -- hence these ugly detours...

I've tried to do the right thing with the alignment:

- we need a 1MB of nothing at the very beginning for U-Boot and its
environment
- the eMMC version of the microSOM has 8GB of storage
- it probably makes sense to split that into two halves for
future-proofness, so that we don't have to deal with overlapping
partitions later
- finally, the 1MB gap between the end of cfg-A and the beginning of
rootfs-B is there to be symmetric with regards to the U-Boot's reserved
space in the first 1MB

Hopefully, we should be able to just (more or less) `dd` stuff in-place.

There's one missing piece: nothing explicitly puts stuff to /cfg/etc/,
yet.

Change-Id: Idc540ecfedf1b17615b01f28cdce5dca9b976260
diff --git a/board/czechlight/clearfog/patches/u-boot/boot.patch b/board/czechlight/clearfog/patches/u-boot/boot.patch
index 1d4520b..f826cb3 100644
--- a/board/czechlight/clearfog/patches/u-boot/boot.patch
+++ b/board/czechlight/clearfog/patches/u-boot/boot.patch
@@ -88,7 +88,7 @@
 +CONFIG_WDT=y
 +CONFIG_WDT_ORION=y
 diff --git a/include/configs/clearfog.h b/include/configs/clearfog.h
-index 5061f6c6fd..2465b3d933 100644
+index 5061f6c6fd..592c7cc499 100644
 --- a/include/configs/clearfog.h
 +++ b/include/configs/clearfog.h
 @@ -32,6 +32,11 @@
@@ -143,7 +143,7 @@
 +	"    if test ${BOOT_B_LEFT} -gt 0; then" \
 +	"      setexpr BOOT_B_LEFT ${BOOT_B_LEFT} - 1;" \
 +	"      echo \"Found valid slot B, ${BOOT_B_LEFT} attempts remaining\";" \
-+	"      setenv rauc_part 2;" \
++	"      setenv rauc_part 3;" \
 +	"      setenv rauc_slot B;" \
 +	"    fi;" \
 +	"  fi;" \