prd-flash: require u-boot-fw-utils optionally
prd-flash: require u-boot-fw-utils optionally

Add requirement for u-boot-fw-utils only when the bootloader is
u-boot.

Signed-off-by: Jens Rehsack <69d49c504a9219a55b8be0f905e8fe3c32ffb57f@netbsd.org>

rehsack/meta-jens,rehsack/meta-jens,rehsack/meta-jens