balena-image-initramfs: Increase initramfs max size

With the move to kirkstone the initramfs size has increased. This increase
should not reduce too much the available space in the rootfs partition
where the initramfs bundled kernel is located.

Signed-off-by: Florin Sarbu <florin@balena.io>

layers/meta-balena-fsl-arm/recipes-core/images/balena-image-initramfs.bbappend

@@ -6,3 +6,6 @@ PACKAGE_INSTALL:remove:nitrogen8mm = " initramfs-module-recovery"
 PACKAGE_INSTALL:remove:nitrogen8mm = " initramfs-module-migrate"
 
 PACKAGE_INSTALL:remove = "mdraid"
+
+# increase the allowed initramfs size
+IMAGE_ROOTFS_MAXSIZE:imx8mm-lpddr4-evk = "51200"