uboot-imx

u-boot.elf: remove hard-coded arm64 flags

This is needed in order to allow building it for other archs.
Move relocation comment to a better place.
Remove no longer needed dts FIXME.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>