1.It make sense to let Toolchain.defs give the default value
2.The board can still change if the default isn't suitable
3.Avoid the same definition spread more than 200 Make.defs
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ic2649f1c7689bcf59c105ca8db61cad45b6e0e64
b329e2377d