arch/m68k: get rid of BR2_GCC_TARGET_TUNE

On m68k, we are passing the exact same values of BR2_GCC_TARGET_ARCH
and BR2_GCC_TARGET_TUNE, which is redundant. Therefore, this commit
removes the usage of BR2_GCC_TARGET_TUNE on m68k.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>