mvebu: mamba resize kernel to 4MB

mamba has a 3MB kernel partition as specified by the DTS.
3MB is not sufficient for building with many kernel modules or newer
kernel versions.

mamba uboot however as set from factory will load up to 4MB.
This can be observed by looking a uboot log:
	NAND read: device 0 offset 0xa00000, size 0x400000
	4194304 bytes read: OK
and from uboot environment variables:
	$ fw_printenv | grep "pri_kern_size";
	pri_kern_size=0x400000

Resize the root partitions from 37MB to 36MB to let kernel expand
into it another 1MB.
And set kernel target size to 4MB.

Lastly add a compatibility version message: kernel-size-migration.
And set it on mamba to prevent sysupgrading without first reinstalling from
a factory image.

Signed-off-by: Tad Davanzo <tad@spotco.us>

target/linux/mvebu/cortexa9/base-files/etc/uci-defaults/05_fix-compat-version

@@ -0,0 +1,10 @@
+. /lib/functions.sh
+
+case "$(board_name)" in
+	linksys,wrt1900ac-v1)
+		uci set system.@system[0].compat_version="2.0"
+		uci commit system
+		;;
+esac
+
+exit 0