boot: provide CONFIG_BOOTMETH_QFW Kconfig parameter

U-Boot is often used conjunction with QEMU to boot via EFI or syslinux.
Here the QFW boot method is not needed.

At least for qemu-riscv64_smode_defconfig the kernel parameter is used
to specify the U-Boot binary. Trying to run U-Boot as a kernel makes
no sense.

Provide Kconfig parameter CONFIG_BOOTMETH_QFW to decide if the QFW boot
method shall be provided.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>