Add initial support for a13_olinuxino board
with below features
- U-Boot 2017.05
- Linux 4.11.5
- Default packages from buildroot

Signed-off-by: Chakra Divi <chakra@openedev.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
[Thomas:
 - use full name in DEVELOPERS file
 - remove parametrization of the post-build.sh script, just hardcode
   the boot.cmd file used as input
 - add missing dosfstools and mtools host packages in defconfig,
   needed because a vfat partition is defined in the genimage.cfg
   file
 - minor tweaks to readme.txt file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>