u-boot: only load the ramdisk whenever needed
Change the list of u-boot commands to send according to the boot command. For "nfs" for instance, it's of no use to load the ramdisk into memory as the linux is booted with root=/dev/nfs. Hence only load the ramdisk in memory for "nbd" and "ramdisk" commands.
initrd_high is also set only when loading a ramdisk.
This is also shortening the list of commands to send, making the boot a bit faster.
Signed-off-by: Rémi Duraffort email@example.com