vexpress64: juno: add alternate kernel and device tree filenames

vexpress64: juno: add alternate kernel and device tree filenames

The latest Juno firmware stores the files in NOR flash as "norkern" for
kernel binary, "board.dtb" for the device tree binary.

The "old" firmware used the name "Image" for the kernel binary and
"juno" for the device tree binary.

Rather than just change the default U-Boot configuration to use the new
names, breaking users with the old firmware, attempt to load the default
filename first.  If that fails, attempt to load the alternate filename.

I've echo'd that we are loading the alternate file to counter the
output from "afs load" shown if the first load attempt fails.  For
example, I see output like this on my Juno board when it's configured
the with the "old" firmware:

    image "norkern" not found in flash
    Loading Image instead of norkern
    loaded region 0 from 08500000 to 80000000, 00AB6318 bytes
    image "board.dtb" not found in flash
    Loading juno instead of board.dtb
    loaded region 0 from 0A000000 to 83000000, 00003188 bytes

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>