- add initial index for toradex boards reST documentation
- add initial colibri_imx7.rst doc file which provides all needed
information for obtaining a workable image ready for flashing
for both eMMC/NAND versions of Colibri iMX7.

Signed-off-by: Igor Opaniuk
Reviewed-by: Bin Meng
Reviewed-by: Oleksandr Suvorov
Tested-by: Bin Meng
[bmeng: make title underline the same length as the title itself]
Signed-off-by: Bin Meng