fdt: Add -q option to fdt addr for distro_bootcmd

distro_bootcmd uses this construct a few times to test $fdt_addr_r,
and fall back on $fdtcontroladdr if not set/invalid:

    if fdt addr ${fdt_addr_r}; then
        ...
    else
        ...
    fi

If the `fdt addr` test fails, it prints the following  message on the
console, suggesting there is an error when there is not:

    libfdt fdt_check_header(): FDT_ERR_BADMAGIC

To remove this potentially confusing error message, this patch adds -q
as a 'quiet' option for fdt addr, and uses this flag in
config_distro_bootcmd.h

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>