minnowboard-max: defconfigs: use silvermont x86 variant

As that is the CPU core in the E38xx Atoms used on the Minnowboard max.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
diff --git a/configs/minnowboard_max_defconfig b/configs/minnowboard_max_defconfig
index f7ba26e..9495bf3 100644
--- a/configs/minnowboard_max_defconfig
+++ b/configs/minnowboard_max_defconfig
@@ -1,6 +1,6 @@
 # Architecture
 BR2_x86_64=y
-BR2_x86_atom=y
+BR2_x86_silvermont=y
 
 # Misc
 BR2_SYSTEM_DHCP="eth0"