Make FIT support really optional

Due to some mistakes in the source code, it was not possible to really
turn FIT support off. This commit fixes the problem by means of the
following changes:

- Enclose "bootm_host_load_image" and "bootm_host_load_images" between
  checks for CONFIG_FIT_SIGNATURE, in common/bootm.c.

- Enclose the declaration of "bootm_host_load_images" between checks for
  CONFIG_FIT_SIGNATURE, in common/bootm.h.

- Condition the compilation and linking of fit_common.o fit_image.o
  image-host.o common/image-fit.o to CONFIG_FIT=y, in tools/Makefile.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
[fabio: adapt for 2016.07]
Signed-off-by: Fabio Estevam <festevam@gmail.com>
[Ricardo: fix conditional compilation and linking of the files mentioned above
for 2016.07]
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Jörg: adapt for 2019.01]
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
[Retrieved from:
https://git.buildroot.net/buildroot/tree/package/uboot-tools/0003-Make-FIT-support-really-optional.patch

]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>