uboot-tools: fix FIT support and make it optional

uboot-tools: fix FIT support and make it optional

Fix several issues regarding the support for Flat Image Trees (FIT).

- Add a patch to really allow turning FIT support on/off, which was not
  possible due to bugs in the code and in the tools Makefile. This patch
  has been sent upstream but not applied there, yet.

- Use independent options to control FIT support on host and target
  packages.

- Subordinate FIT signature support to the activation of FIT support, in
  the target package, not to mkimage installation.

- Add a dependence on the dtc utilities because mkimage needs it when
  FIT is enabled; otherwise mkimage fails like this:

    $ mkimage -f firmware.its firmware.im
    sh: dtc: command not found

- Add BR2_PACKAGE_UBOOT_TOOLS_MKIMAGE_FIT_SIGNATURE_SUPPORT to the
  Config.in.legacy file.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>