x86: add intel microcode entries to grub config

Create initrd enries for x86 images, that'll load intel microcode as
early as possible. To achieve that the test module for grub is enabled
which provides shell-like conditionals. Also restrict the late load of
microcode to AMD processors.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>

target/linux/x86/base-files/lib/preinit/02_load_x86_ucode

@@ -2,8 +2,10 @@
 # Copyright (C) 2018 OpenWrt.org
 
 do_load_x86_ucode() {
-	if [ -e "/sys/devices/system/cpu/microcode/reload" ]; then
-		echo 1 > /sys/devices/system/cpu/microcode/reload
+	if grep -q AuthenticAMD /proc/cpuinfo; then
+		if [ -e "/sys/devices/system/cpu/microcode/reload" ]; then
+			echo 1 > /sys/devices/system/cpu/microcode/reload
+		fi
 	fi
 }