build-oe-binaries.sh: update ramdisk to use new cpio format

The old ramdisk-oe.img format is causing issues on 5.0 kernels in which
initramfs errors are no longer ignored:
ff1522bb7d98 ("initramfs: cleanup incomplete rootfs")

Update cpio command to use newc format, the new (SVR4) portable format,
which does not cause issues on recent kernels.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>