mediatek: switch to use seperate ramdisk for initramfs images

mediatek: switch to use seperate ramdisk for initramfs images

MediaTek targets always use U-Boot's modern uImage.FIT format which
allows bundling several blobs into a single file including hashes,
descriptions and more. In fact, we are already using that to bundle
the Flattened Device Tree blob with the kernel on this and many
other targets.
In the same fashion, we can now make use of the newly introduced
support for building seperate ramdisk to uImage.FIT with a dedicated
initrd blob checked and loaded by U-Boot instead of embedding the
cpio archive into the kernel itself.
This allows for having larger ramdisks, choosing ramdisk compression
independently of kernel compression (while only kernel is decompressed
by the bootloader) and for more easily replacing or modifying the
filesystem contained in an initramfs image.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>