cmd: fdt: Use separate CMD_FDT Kconfig entry instead of OF_LIBFDT

Create CMD_FDT Kconfig entry to have an option to disable fdt command
which is not required for small configuration which requires libfdt
only.
Enable it by default for all targets which enables OF_LIBFDT.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
[trini: Fixup flea3/sandbox/id8313/siemens-am33xx/smartweb]
Signed-off-by: Tom Rini <trini@konsulko.com>