x86: add intel microcode entries to grub config

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>
(backported from 546fced2a23557e95dd34246744c3aa6cad92fe6)