config/odroidc2: force usage of gcc 4.9.x

config/odroidc2: force usage of gcc 4.9.x

due to an issue with odroid's u-boot, compiling u-boot with
gcc 5.x will result in a non booting image.
so a gcc 4.9.x compiler must be used.
this solve bug #9326

thanks to Erico Nunes <nunes.erico@gmail.com> for pointing
to the right issue.

Reported-by: <gahabana@mail.com>
Signed-off-by: Dagg Stompler <daggs@gmx.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>